I downloaded Osu! yesterday, but I haven't been able to play because whenever I try, it enters a loading screen and says "Processing Beatmaps" at the top, but it doesn't finish. The longest I've waited is around 40 minutes.
Close osu!, find a file named "osu!.db" located inside your osu! folder and remove it. Restart your game and head to the song selection screen, you should see your maps start to reprocess one more time. It might still take a bit depending on how many maps you have, but hopefully it should finish.
Otherwise it will just be called "osu!" and the type of file will be "Data Base File", this can get confusing because there will be another filed named "osu!" but it's a configuration file that you don't really want to delete. The osu! executable file will also just be named "osu!". It's best to just avoid this confusion entirely.