Hi, PiersG_99,
Sorry to hear you're experiencing this issue.
The error "select the proper boot device or insert boot media" could be caused for two reasons, has a problem with the hard drive and is not recognized on Bios, or the boot files were corrupted. The first thing that you need to check is if your hard drive is recognized in Bios and if doesn't have any problem.
Here is which key you need to press to enter on Bios:
Acer: F2 or DEL
ASUS: F2 for all PCs, F2 or DEL for motherboards
Dell: F2 or F12
HP: ESC or F10
Lenovo: F2 or Fn + F2
Lenovo (Desktops): F1
Lenovo (ThinkPads): Enter + F1.
MSI: DEL for motherboards and PCs
Origin PC: F2
Samsung: F2
Sony: F1, F2, or F3
Toshiba: F2
Once you are on Bios settings, go to the boot tab and check if your hard drive is recognized, if it is not you need to check the hard drive connection.