After adjusting something under the playfield with power on, no switches work.

Switch matrix dead after a screwdriver slip under the playfield

Cause: A coil lead (50 V) was shorted to a switch lead. This blows the switch matrix fuse (F115, F101 on WPC-95) and usually destroys CPU board U20 (ULN2803), often U14 (74LS374; U23 on WPC-S/WPC-95) as well, and can kill LM339s on opto boards.

Checks

• Check F115/F101.
• Test or substitute U20 and U14/U23.

Fix

• Replace the fuse, U20 (socketed from WPC-S on), and U14/U23 if needed; check opto board LM339s.
• Work under the playfield with the power off.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Switch matrix chips on the CPU board
• Why CPU U20 fails and how to replace it
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Lamp matrix faults with all lamp transistors good
• BR1 (D11-D14 on WPC-95) and the board around it scorched
• One switch does not work
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)