Hi. I'm running a Planescape campaign. The players lost a bit of their memory by taking part on a party which ended with everybody death, except them.
The party was on a demiplane which self-destructed. Now they are looking for restoring the memory (they lost 3 - 6 months) in Sigil.
Is there any lore or home fiction people, seer, item, potion, living dream which I could use? I'm looking for at least three option.
Thanks in advance.
I guess it really depends upon how they lost their memories. They might try going to the Gatehouse to seek some kind of counseling or to determine what is wrong with them. (Mind you, they may not want to take this choice ... the Gatehouse can be a scary place)
They could visit the Sensates, if any of the party are affiliated, and see if someone there could help them restore lost sensations or memories ... I imagine there is someone so skilled in the faction, considering the loss of one's experiences would be seen as a terrible fate. (if any of them are Sensates it is quite possible they have left at least a few previous experiences in the sensory stones - which they could then find and relive)
If the loss is a result of some curse or foul magic I have, in my game, an npc called Hervallion the Hexbreaker - a halfling who makes his living researching the curses people labor under and determining how to release them/actually going through the effort to arrange such. (for example: if someone were cursed to lose their memory until they once again looked upon the face of a man they'd killed, the Hexbreaker would discover which plane the spirit had gone on to, locate the individual and arrange a meeting ... or, if rare ingrediants were required to remove a curse, Hervallion would appropriate them.) His charges are based on the power and nature of the curse, not to mention any personal injury he may suffer in the process. Naturally, he requires a sustantial down payment, but doesn't charge more if he fails to fix the problem.
A restoration spell might return lost memories - dependant upon how it is worded in the game edition you're using.
I have another npc half-Illithid (he is a human who managed to fight off his transformation into a Mind Flayer, though he only triumphed after part of his body had already been transformed - thus leaving him with a large eye, tendril stubs and three fingers, all on one side of his body. The actual term for this, very rare, type of person appeared in the Illithiad, though I don't have that handy at the moment) who makes his jink by doing something similar to what the PCs need ... he invades the minds of his clients in order to unlock repressed or forgotten information; to break charms, allow people to face their inner demons, oust posessing spirits, etc. But, there is a chance he will drive the client insane in the process. (plus, he's creepy as all hell)
Being that this is Planescape, the only limit is your imagination ... I hope these ideas gave you something to work with.
Will use Legend Lore.
Will use the information some female Dwarf sigil born has some potion that can induce living dreams which could touch some places where memories are sent and never erased.
I'd like to have a third option too. There are three players and things happen on threes...
The potion idea is fine ... though I hope you'll make the npc interaction interesting. Legend lore seems like a bit of a cop-out to me ... I guess it depends upon how major a part of your campaign you want this to be.
Remember, in Planescape the more bizarre and interesting an experience, the better.
Jack of Tears, very nice ideas, I guess I'm gonna stole one of your npcs.
By the way, is your name from that nice Scarred Lands fellow?
Oh, you can be sure... let's say this dwarf just can't remember when she gets that potion... one of the players will have to drink it and, bad luck, she forgot about the part "something is back, something is gone".
I'm probably trowing an aspect of Morpheus (hello Sandman) in disguise. Their story being told somewhere else, using other characters chosen from the player who drink the potion paradigm.
English's not my mother language, sometimes I can't turn my exact thoughts on words.
Actually, Legend Lore, if done right, can prove very interesting indeed, especially if, for example, the spell only reveals a couple of small, seemingly unimportant and unrelated events which will later come into their own as major plot hooks.
See Clueless for details.
Or.
It can blow your brain out the back of your skull and leave you twitching on the ground for a few hours. Or maybe that's just me when I do Legend Lores with heavy magic on Keeper gear... 
The most direct way is to find someone who can manifest the 9th level Psion (Telepath specialty) Power, Psychic Chirurgery which heals just about any form of mental damage.
Another more plot-driven method would be to track down a bunch of people who remembered what the characters did during those months, and bring them together with a ritual using memory crystals drawing out their recollections of those months together. There might be some inconsistencies in your memory with that method.
Or seek out the memories themselves hidden away somewhere in a dreamscape in it's manifested form. Once you find those memories you must consume them somehow to become whole again.
Or perhaps they were somehow taken away in the River Styx, there's this place in Baator on the River Styx where memories are brutally slaughtered. In theory you could try to save your memories from being butchered there.
After reading your posts I concluded that in fact their memories were destroyed and used by Someone to create Something. It can't be restored... but the Planes have some very interesting places such as dreaming (from my former example). Every dream is stored and can't be erased.
Every other spell and method will reveal just part of a puzzle they'll have to figure out.
Try the spell Legend Lore, it should help.