Hooked up GFI on one circuit. Everything appeared okay and then the next day, On a separate circuit I lost the ground. If I test with the black and green, then I will have voltage. But black to white, I just get continuity!

Sounds like you lost the neutral and not the ground, It is not the GFI tripped as it would disconnect the hot as well as the neutral.
If you get continuity between black and white, then you likely have voltage between neutral and ground.