Tried changing the batteries yet, and have you given the battery holder a close inspection. Those factory holders fall apart on a fairly regular basis.

Tried rapidly turning the power off then back on?

With the coin door open, what do the CPU board LED's do when you hit the diagnostic button?

-Hans

Did this happen when you changed the batteries? Can you advance through the diag settings? If it lost it's settings, you might just need to set everything (including free play). I just did this and I thought I screwed up for a second until I realized I just needed to reset all the values in settings.

http://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6

If you're using pinside to get help fixing machines to flip you should probably consider making a donation

Open the coin door and make sure the coin door switch is not stuck. Flip the power switch quickly a few times and see if it goes into attract mode.

If it goes into attract mode you probably have battery problems.

If it stays stuck in audit the 5101 ram is likely bad.