Stuck in prison !

[Som1Lse]

The easiest way to find is to go back to the starting area (elevator room).
Then take the only way out of it and when you get to the stair turn left (do not go upstairs).
You should see a corridor and at the end of it a door. Do not think about that door as you can't unlock it.
Instead focus on the two cells there are about halfway.
The right one (as in the one to the right) has a hammer hidden under the bed. (or well not exactly hidden)