I just changed the water in my hot tub, and now when I power it up to heat the water it runs for a few seconds the GFCI trips and power is lost. This has happened before and seemed to resolve itself after a few tries, but not this time. The hot tub had been operating perfectly well before I changed the water.

Check all of you wiring connections for moisture or loose wire nuts. It could be that the GCFI is doing its job. If you can get your hands on a meter it would be helpful to measure the load being pulled by the pump and the heater.

How old is the spa? It's not unusual on an older spa, with a weak heater, for the heater to go out upon a water change due to the "shock" of either the cold water, or the initial start up of getting any air out of the lines.