Andrej,

In addition to the above suggestions, 1st thing I would look at is the coin door interface board.  With the game off and unplugged for safety, remove the playfield glass and lockdown bar.  Next remove ALL balls (to save breaking playfield plastics) and slide / lift up playfield.

On the coin door there should be a black covered wiring loom (ie black plastic tubing) which goes from the coin door loom to a small PCB (circuit board) on the inner left hand side of the cabinet.  There will be a white plug on the end of the loom which plugs into the small PCB.  Just make sure that is connected OK and maybe just reseat it.

Is the rest of the game playing OK?

Did you purchase the game from a commercial dealer?  If the problem is on the CPU board (ie battery damage), maybe contact them before digging into it.