What do I do if I forget my lines on stage?

If you know roughly what the line is, find a way to weave in the basic idea. Otherwise, if you know that one of your fellow cast members has a line to say soon after the one you forgot, find a way to prompt them to say that to get past it. Standing there silent for any longer than a few seconds will tip the audience off that something was missed, so you want to resurrect the situation one way or another quickly so the audience does not realize anything was off. Learn your cues - it's all well and good knowing your lines, but if you don't know where to say them, there's no point.