Most hard disks and PCs have finite lifespans so it's not uncommon to see a machine suddenly forget something on the hard disk. It's all part of why you see many write and talk about BACKUPs and keeping all they need ready to reload a machine.
In your case you have your car but no keys.
Too bad you didn't run BELARC ADVISOR (noted in these forums) and keep that print out.
Bob
I have a clone with Win98-SE - for years. After my desktop froze, I did a CNTR-ALT-DELETE to restart; I've done this a million times. In the middle of the reboot (after the Windows 98 light blue cloud screen) I got an "i" message box that said my Soundblaster MIDI driver would not load, and behind that box was the first dialog box for the Windows 98 installation wizard. I clicked off the Soundblaster error box, and the Windows 98 installation wizard will not allow me to proceed without entering the product key. I have the disc but the product key is not listed on the outer plastic case. Do I have to entirely reinstall windows 98? Is there a work-around? HELP!!!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic