From Flyingfrogwiki.com

Last Night on Earth, The Zombie Game Game by Jason C. Hill

Locked Door

Play this card when a Hero tries to move through a Door space. The Door is locked and the Hero's move ends in the space before the door.

"...Damn!" "Does anybody have the key?"

Notes from Board Game Geek

Q. How long do locked doors stay locked? Just a turn until a Hero finds keys? Would it be possible for Heroes to become locked inside a building?

A. ‘Locked Door’ only affects a single character for their current move. It is assumed that after that they (or any other character) are able to force the door open or break through somehow.

http://www.boardgamegeek.com/article/1699929

Locked Door only lasts for a single Hero's movement (preventing them from moving through the door and ending their move early). The idea is that they can quickly force their way through or find an alternate way (smashing glass etc), it just takes a little longer. Ending their move represents the extra time it takes them.

http://www.boardgamegeek.com/thread/216807