Please help! Unity don't work. 400+ CS0246 errors

I started using Unity with a friend, but one day my PC was freezing while starting Windows. After I did a System Restore, my PC Work fine.
So I started Unity again and now there are 400+ Errors.
I reinstalled Unity two times, but it is still doesn’t work.
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Is this project youre having issues with completely untouched, i dont really have the qualifications to give adice on this topic, but this error tells you that youre missing a namespace like using UnityEngine;. So its could be an issue with a custom script or a downloaded package or asset. other than that i have no idea