Often the latch has been closed and the door won't catch until the latch is popped back. Use a screwdriver or other tool, make sure the door is unlocked then operate the inside door handle and open...
If it is the latch that is sticking lubricate it or replace it.
If the door sticks on the latch side:
Plane down the latch side or if the screws on the hinges are loose tighten them, if necessary...
Has the door been hit? If so, a body shop would be the first place to try. If not, roll the window down and spray penetrating oil, like WD40, into the latch area of the door (this is about at the...