This should take about 5 min total.

First  start with camera in a clean work area.

Remove 2 bottom screws and then remove bottom.

Mirror cocking arm is on the left side of camera. For most jam's this is where the problem is.

Grasp bottom of cocking arm with either a pair of small pliers of you can even hook it with a small screw driver and pull it to the left.

You should hear a click when the mirror assembly cocks.

After this check to see if you shutter fires. Then replace bottom plate.

So what causes this type of problem. Mainly improper cocking of the camera. The camera must be completely cocked to the end of its cycle or the shutter may cock and not the mirror. Use steady force when cocking the camera and do not force it. but in a smooth steady stroke cock the camera fully. Crank lever should not be flicked.