Yeah, it sounds like having the power on when there's no water inside will at the least damage it and possibly cause a fire. Do you have no one you can call who is handy? At this point it might be worth hiring a handyman.

Did you look at troubleshooting in the guide? I searched through for power and fuse, but didn't see any way to turn it off except your breaker/fuse system external to the heater.

Edit: I noticed on page 19, top of the page, the ECO button. Read about that. That might be the problem. Probably worth spending more time with the manual if you don't call someone handy.