A Microsoft file hosting and synchronization service.
Since you can sign in at onedrive.com, your account and password are fine, so I would treat this as a problem with the OneDrive app on the PC, not with your account.
First I would check whether OneDrive is just hidden. I would click the small up arrow in the taskbar corner to show hidden icons and look for the OneDrive cloud icon there.
If it is not there, I would reset OneDrive. I would press Windows key + R, paste %localappdata%\Microsoft\OneDrive\onedrive.exe /reset, and press OK. Then I would open OneDrive again from Start and sign in. Resetting does not delete your files, it only disconnects the sync connections, so OneDrive will sync again after the reset.
If the reset command reports that Windows cannot find OneDrive, I would try C:\Program Files\Microsoft OneDrive\onedrive.exe /reset instead.
Microsoft describes the reset steps here: https://support.microsoft.com/en-us/onedrive/reset-onedrive
If OneDrive still does not start after the reset, I would uninstall it from Settings, then Apps, then Installed apps, and install it again from the Microsoft Store.