Character Sheet Half-Reset Randomly?

I was looking at a character sheet in a game I'm a player in when the site had to refresh. When I came back I saw that the character's stats had been completely reset: ability mods, levels, skill points, hp, all gone. All his feats, features, and equipment were still there, but the rest was reset to the defaults for each stat. Is there any way I can get back the stats he had before?

Unfortunately no.  Roll20 actively saves your work as you put it in, but that doesn't help if there is an internet disruption between yourself and Roll20. Just one of the pitfalls of working online. It sounds like you weren't actively connected to the website when you were making changes and so they didn't take.