It is not unusual for an older game's on/off switch to wear out and break.
You can take these apart and rebuild them, or just replace them. A good
preventative measure is to use a power strip, so you don't have to use the
switch at all. Some machines such as Stern's Orbitor 1 have a
quadruple-throw switch that you will have a tough time replacing, and a
power strip effectively solves that problem. Also, machines older than the
late 60s didn't have on/off switches, so you need something like a power
strip. I recommend a power strip with a lighted switch so you can tell if
the strip is left on.