There could be several possible reasons for this issue:
Door Latch Issue
The part that keeps your door shut is the door latch and if it get stuck or broken the door won’t open.
Broken Handle
If the inside parts of the handle are broken pulling on it won’t release the latch that keeps the door closed. Both the inside and outside handles could have this problem.
Lock Issue
Sometimes the lock doesn’t work right. Nowadays most car has electric locks and if something went wrong with the electrical system you wont be able to open the door. In older models It is a manual lock and probably some internal parts are broken.
Frozen Lock
In cold weather the internal parts inside the door can freeze and stop moving. Probably they are not lubricated enough and got stuck.
Door Misalignment
If your car door or its frame is bent as a result of an accident the door might not fit in the frame properly and can make it impossible to open.
Disconnected Linkage
There are rods inside the door that connect your handle to the latch and if these rods come loose you won’t be able to open the car’s door.
