What does install proper boot device or install boot media in selected boot devi

For it to happen so suddenly, I suspect a hardware failure:
A bad cable from the HD to the MB (replace cable to test)
The HD has failed (remove HD from tower, put it in a $20 external USB enclosure and plug that into a working computer to see if it reads (Optional: if it does, download Seatools [google it] from Seagate [works on most HDs] and test the HD for errors)
OR, if the HD works, then Windows may have been corrupted and you should reinstall it with your OEM disc. If the HD works and you don't have your Windows disc, then,
you're probably screwed.
If the HD works and Windows is intact, and it's not the cable, then the Motherboard has likely failed or BIOS may have become corrupted and you should look at it and see where it is looking for the Boot Media, which should not happen for no reason unless your power supply is going bad and throwing voltage spikes (which can corrupt BIOS, corrupt Windows, cook a HD or any other piece of hardware), you can try reflashing you BIOS but I'll bet that's not it.
If there is anything here you don't understand, you can google it and learn. Chances are your HD has gone south or Windows needs repaired.

any time you ask for assistance please post your specs