Hmmm. I would make sure you have a monitor connected in the graphics and not in the motherboard. Or try going to the search, write the graphics settings, browse and go to the program files - epic games - fortnite - fortnitegame - binaries - win64 and select everything with the fortnite logo. Then you give options and you give high performance. I don't know anymore, an expert would have to: D
Fortnite has an issue with the current "season" that they are aware of. I don't know if they will fix it. I can tell you another had this very issue and the OP had the same CPU. The OP went to a 5900X and the problem went away. Reason is it's a CPU bottleneck that you can't fix running a 3000 series CPU. Even a 5600X would most likely cure your issue. The 3950X was never considered a good gaming CPU, it's far more geared toward work type apps, AMD will tell you that. As the 5950X is much the same, better for work than play.
Now, you do say it was 250-300 FPS an hour ago, using that card and suddenly you're getting 60 FPS. So it's the Fornite bug and it's on their end whether it's the server capping FPS or dev's messing things up. I can assure you it's not the card per se. I have the non-XT version of your card and notice FPS is not so hot in CPU intensive games, Fornite would be one but I don't play it. Many. many complaints about Fortnite on multiple forums lately, even Nvidia.