If it's just closed the door the credit card shimming may work, but if it's completely locked then be careful or you might break you credit card too.
(
or youtube it and look for how to open door with credit card.
)
It all depends on the latch. If you can exert force on the inclined part you might force it open, if you can't reach it, than go to what the other suggested:
Call a locksmith or take hinge pins out
Anyway be careful.
(and needless to say only open locks if they are actually yours.)