How cold is it in the room your PC is in? I had this happen once with a PC in a basement room in winter that got below 60 f. How old is the drive? If it's under warranty, I would try to save the data and get it replaced ASAP.
IDE drive with ATA controller. On bootup drive does not always spin up. Sometimes get 1 beep 3 or 4 times then reverts to bootup from secondary drive.
Always spins up eventually.