How good is the lock? Is it a dead bolt? If it's not a dead bolt and the lock wasn't installed with a security plate you can try sticking a credit card in to depress the latch inside.
Discussion
I tried the creditcard but failed :(
it should work though I was told.
I waited 5h until my friend with a key came :)
Bummer. If it was installed correctly it won't work. There's a little security plate.