Either a failed hard drive or a badly corrupted OS. Your post has so few details that I can't tell what it could be.

Let's tackle it both ways?

1. Bad drive. Replace the drive and use the restore media to bring the machine back to service.

2. Bad or damaged OS. Use the restore media to wipe out the machine and bring it back to service.

I won't write how to use HP's restore media since HP does that. No need to duplicate HP's web pages.
Bob