My 90 automatic bus suddenly won't start. It's had issues with the neutral safety switch but has at least always started in neutral. I drive it yesterday without any problems.  Today though, not even a click from the starter. I've shifted it from part to neutral a million times and tried in both positions. Alas, no luck. 
I have never dealt much with automatics so have no idea where to start. Do the safety switches fail completely? Can they be bypassed?

The batter is good and even with a buddy jumping it, no luck. 

Thanks!

-craig