Know who knows how to fix everything and when you can't find him/her, who else might know
Use helpers in classes with older children. They may know more than you do and can set up while you are checking roll, etc. This is particularly helpful when you have one class right after another with no break.
Keep talking while you are pondering a technical difficulty. Silence is a vacuum which children will fill. For a short pause, quiz them about what you just talked about, review other ongoing material or give them a think question about what you will study next. If your attention is too distracted, have a student lead an a cappella song or an echo clapping exercise.
Make backups of important accompaniments. For programs, always have a backup with you. For classroom, backup often and have alternatives available.
Always have an emergency lesson available which requires no electronic equipment. This could be anything from a game (Music Bingo) to requests for favorite songs (you will have to accompany or sing a cappella), or even a story you could read to them about a famous composer, etc.