Who drained the tub? You or someone else?
Why was it drained?
If you did not drain the tub, could the other person have unplugged something? Or turned off something behind that hidden compartment where the pump and so forth is? Tripped the GFCI? (Press "Reset")
Also try turning the breaker(s) fully off, then back on. There could be a breaker at the hot tub as well as another at an electric panel at the house.