Where did the partion go???

I will add the type/model of machine and hard drive in question at a later date. The pc is running win98 on a pentuim 150 with a 18 or 20 gig hard drive that is partioned into 2 drives… C and D. The D drive disappears and will not reappear if adefrag is run on the c drive. Scan disk can be run on both c and d prior to the defrag without any errors. After Defrag the only way to get the d drive back is to delete and recreate the logical in Fdisk. Someone PLEASE help me on this…..