Why is my internal hard drive not recognized anymore?

Since yesterday I have the problem that my notebook does not recognize the HDD.

What can I do now? So there really is nothing left on the PC except Windows, because Windows is on the SSD. When I start the PC the message "D: / Libraries / Desktop" is not available. On the hard disk were important files. I urge for help!

Thanks in advance.

Edit: I have restored the disk in the disk management and now the disk is empty. What now?

First of all, you look into the disk management. Then check the connections.

Sorry, I can't follow you.

For example, What does the PC have to do with the notebook?

I always wanted to say it:
No backup → No pity

I constantly preach that you need a backup, no one does it.

Is the hard drive because if you disconnect the PC for a short time from the power (If possible, remove the battery in the notebook) and reconnect? If not, then the hard drive is gone and the data gone. One can try a data recovery over a specialized enterprise, which become however very fast very dearly.

Disk Management does not display the disk

I confused I meant my notebook, that has a problem

Ahh now the hard disk is displayed in the volume management, but there's: not assigned