A Microsoft file hosting and synchronization service.
Hi,
Do you get any error code or is it only the loading screen?
This usually means a local cache or Windows profile issue, so the direct fix is to reset OneDrive using Win + R then run %localappdata%\Microsoft\OneDrive\OneDrive.exe /reset and reopen it, then check Settings, Accounts, Email and accounts and remove then add your account again, and clear OneDrive entries in Credential Manager before signing in again.
If it still hangs, test with a new Windows user profile since that confirms if your current profile is causing the sign-in to stall.