Hello,
We recently troubleshoot someone on a French forum (flipper.fr) who had a similar problem on a Jacks To Open, so the symptoms speak to me!

Since the displays show "000000" and there is nothing on the credits it means that the CPU board boot well, but that the pinball is in TILT mode. This is not the SLAM mode, because in this case, there is a wave effect on the displays. But as the displays are stable, then it is that, it is in permanent TILT.

On the 80 and 80A systems, if the TILT switch (SW57) is detected at startup, the displays show "000000" fixed and the credit display remains off. The controlled lamps remain off, there is no attract mode. In addition, it is not possible to use the service switches (Sx7), so you can not add credits (SW17, SW27 or SW37), to start a game (SW47), but worse: CANNOT start the TEST mode (SW07).

It is already necessary to determine if the fault comes from the CPU board or the cabinet/playfield.
Disconnect A1J5 and A1J6 and turn on the pinball machine. If there is no change, it is that the fault comes from the CPU. However, if the pinball comes alive (attract mode) is that it comes from contacts (playfield or front door).

In both cases, the problem of permanent tilt very often comes from the RTN7 line of the switch matrix, which is grounded.

If it comes from the CPU, it is often the Z14 or U4 circuits that are toasted.
But it happens more frequently that it is a problem on the contacts. Check the tilt contacts, both the pendulum and the roll-tilt.
You also have to look at the coin mechanism in the door. The RTN7 signal passes through the coin mechanism (SW17, SW27 and SW37), and sometimes a wire touches the ground. This was the case for the Jack To Open that was repaired, one of the coin operated contact with the mass.

But first, try to disconnect A1J5/A1J6 to isolate the problem : CPU or cabinet/front door.

As the problem look solved, it may be an intermittent problem.
Hope this help, if the problem come back.