It would be helpful to post make of PC and the system you are using, how it was ‘turned off’ , and any codes with the messages on your screen. It is running diagnostics, it you can over-ride that but it sounds like that may be above your skill set. Best wait until the PC finishes the diagnostic tasks and this may take a while.
Had similar happen.. it said I had a damaged section on my disc or something similar to that.. it took quite a long time for it to complete its task. All seemed ok afterwards