I have a 240V outdoor spa with a 30A GFCI disconnect nearby. The spa was working fine until a week ago. Totally dead. I had a spa repair come out and he diagnosed that it was due to the input power. He said that in the spa I have 0 volts between the black and red (should be 240v) yet have 120 v from black to white, and also from red to white. The repair person also checked the line from the main circuit breaker to the GFCI disconnect box and found the expected 240 v between the red and black. Does this make sense? Do I need to simply replace the 30A GFCI? By the way, the 30A GFCI test button works as expected and causes the switch to flip correctly.
If the GFI failed in such a way that it completely disconnected L1 or L2 on the input side and then connected L1+L2 together on the output side, and connected those to either L1 or L2 on the input side, then you would measure 0 instead of 240 on L1-L2 and 120v on L1-N or L2-N
Note that this is all entirely theoretical. You have a practical issue. I'm inclined to recommend you get an electrician in which your spa repair place should have done already. A lot can go wrong with mixing electricity and water and this entire thing doesn't feel right.