When my wife tries to switch on her computer there are a couple of beeps and the message:

Primary hard disk drive 0 not found
Primary hard disk drive 1 not found
Secondary hard disk drive 1 not found

What should I do now?

Thank you

Replace the hard drive or drives.

It is not likely all the hard drives failed at once. This sounds more like the controller is bad which unfortunately, points to the motherboard. How old is the motherboard? If more than a couple years, I would probably replace the battery just because they are cheap. When replacing the battery, just remember to go into the BIOS menu, set the date and time, then save and exit. Then see what happens.

If the 12V output of the power supply is a bit low, the drives might not spin up properly too.