If restarting doesn't fix it, you need to flash the operating system, meaning you need to reinstall the operating system. this can be done without losing your files and documents. If that doesn't work then it's a hard disk failure which needs to be replaced, there's no way to do this without losing your files. The hard disk could cost from 3K to 7K. If that doesn't fix it then its a BIOS issue, which is hard to fix here in ET. There are only a handful of places that could fix it. You should only try the first step (reinstalling the operating system on your own) The others should be done by professionals. For further instructions on how to flash the operating system check youtube.