We'll need to get this moved to the General Repair section so you get more help.
I assume this is a Midway Space Invaders, and not a Taito?
I worked on one once that used to give a Tilt error quite often. I eventually found that it seemed to be caused by the +5 getting a little to low. I ended up installing a switcher and bumping the +5 up a bit, and the error stopped happening.
Although, if this IS the first and only time you've seen this, you may check for a slam-tilt inside the coin door. If it's adjusted improperly, too much shaking or a hard bump of the coin door would cause the game to reset. Can't recall if SI has one or not, so check it out...