What to do when I accidentally become invisible.

This appears to be a serious problem in science fiction. Accidentally becoming invisible is very different from becoming invisible on purpose, such as Harry Potter with his cloak of invisibility.
When someone becomes invisible intentionally, usually the person knows how to become visible again. In sci fi, accidental invisibility is usually termed "out-of-phase." When one is "out-of-phase," the person cannot affect the world around her or him. Therefore, eventually the person will die of starvation and dehydration if the problem isn't fixed. This is serious. Somehow, the out-of-phase-ees pass right through all matter, including walls, but do not pass through floors. Physicists are looking into the bizarre properties of floors which make them impermeable to those out-of-phase.

If this happened to you, what would you do? Since I could not affect the world around me, I would not be able to tell anyone that I was still there, instead of the common assumption that I was eaten by a space monster or vanished into a black hole. There seem to be two options in this case. First, I would take a look at the device I was messing with when I first became invisible. I may discover that it has some sort of interface to allow the out-of-phase to communicate with those in-phase. I may be able to see an entire display that those in-phase cannot see. Usually, however, it will be written in an obscure alien language which I do not know. You may want to study up on alien languages in case this scenario happens. The device may allow some sort of direct communication with the in-phase world. In which case, if I can just get someone to look at the device, I'm saved.

In the event the device does not allow any communication, I really only have one option: hope I have smart friends. In general, surround yourself with brilliant people who will be able to figure out and fix the problem should this happen. If possible, make friends with an android. They seem to be good at deduction. Secondly, be consistent and reliable. If you are an unreliable person in general, people will assume you just didn't show up. However, if an otherwise reliable person fails to appear, intelligent people will think that maybe, just maybe, you're out-of-phase. They are your only hope.