If I live in a town house, other tenant was shutting off my power via switch outside her front door, is it illegal? Can I sue them?

Have you spoken to them to find out why they flip the switch. Maybe its one of their children doing it. It is also best to find out why someone is behaving as they are before threatening to sue them. If you do sue, be sure to come up with a list of how what they do has hurt you [reset all clocks, have to turn TV back on, never sure if local power outage, etc.]. Try to get a statement, under penalty of perjury if possible from the electric company as to what caused the power stoppage.