Use a thin screwdriver
If your knob looks as if it requires a key you should slip the head of a thin screwdriver into it. You should then turn the screwdriver on the left in order to unlock the door mechanism.
Make use of your credit card
Here you should slip your credit card in between the door and the door frame then slowly slide the card downwards while at the same time pushing it further from the crack.
When you reach the level of the handle you should work the card into the crack and use the card to push in the metal bolt that keeps the door closed. When you push the bolt you will open the door.
Remove the door knob
If there are no openings in the knob you should consider removing the door knob. You should remove the screws with a screwdriver and remove the knob.
After removing the knob you should reach into the hole in the door and remove the bolt that connects the knob to the hole in the strike plat. After opening the door you should replace the knob pieces the same way that you removed them.