Thor is deemed "unworthy" to hold the hammer after The Watcher (old white guy Nick Fury- don't ask) tells Thor about something he did in the past which causes him to lose the hammer. His hammer just sits there until someone picks up and this is revealed to be a new female Thor. This is explained by the inscription on the hammer stating that
"Whosoever holds this hammer, if he be worthy, shall possess the power of Thor." Then we find out that Jane Foster is Thor (Thor himself stayed around using the name "Odinson" and doing his own things in other comics.) They gave the character a finite life span in this role because she was dying from cancer and every time she changed into Thor, it undid whatever treatment she got.
That's gonna be pretty heavy if this is what the movie storyline adapts. The character then died after
one final transformation, Thor was back to Thor, and they brought her back again recently as a Valkyrie.
It's confusing since I don't follow comics as religiously as other people so I don't get why there would be two people named Thor.
They've done this before- Beta Ray Bill was "worthy" and transformed into Thor after picking up his hammer for the first time.
Comics
Oh yeah, forgot about this-
After Jane Foster Thor, there was also WAR THOR, who apparently was Volstagg wielding the Ultimate Thor's hammer.