Door knob doesn't lock?

My doorknob on my bedroom door does not work. It has a turn lock on the inside and a hole on the outside. The issue is that even when i turn the lock to the locked position, the knob can be easily unlocked by simply turning the doorknob with a little torque. The issue has nothing to do with the doorway because the lock doesnt stay locked when I try it and the door is open too. How do i fix this?