Do the displays come on? Is this a new problem?

You've opened a can of worms, my friend. Those early ss Williams have lots of things that can make them not play or boot. I know, I had a BK that took a lot of work to get it going

But the best thing you can do is start a thread in the solid state repair forum as this is the EM (electro-mechanical, pre solid state) section.

Sometimes you need to open and close the coin door and turn it off and on to get it to boot. It's in the manual some where.

http://mirror2.ipdb.org/files/310/Williams_1980_Black_Knight_English_Manual_with_paginated_schematics.pdf

Theres your manual. You need to turn it off and then on again.

Twice. With the coin door open if I remember correctly.

And have good batteries installed, with no corrosion on the holder.

But either way, you should be able to press Advance inside the coin door to advance through the options. If you're getting stuff on the displays but Advance does nothing you've got another issue

Cant recommend much beyond making sure it's plugged in (1J4 on the MPU), and making sure the buttons are actually working (check continuity across the lugs when you press it). Could always be a board issue but those will be a bit complex to fix

Not sure if this helps, but... My black Knight did this once... the 2.5 amp slo blo for the drop target resets was blown. Replaced it and all is well. It must have blown when I turned power on for some reason, because the last game I played the previous weekend before shutting it off had no problems.