Just to make sure--go to the control panel, open device manager (click System, HArdware, then device manager), and make sure the drive is listed. If it is, double clicjk on it and check the General paned at the bottom under Device Usage. Make sure that the drive is enabled.
But your situation is indicative of something else that is wrong. What did you change before this drive was disabled, and have you scanned your system for viri and trojans?
Drives do not normally disconnect without some help, and finding that will be the real solution to your problem.
The OS is Windows Xp, the Hard Drive is Samsung SP8004H 80.06, the Memory is 512MB. The Drive 'D' was 3.75GB, now it shows 0 GB and as disabled. How can I enable the Drive 'D' again. It seems to give me all kinds of problems. When using the Internet, no problems are seen. Startup is very slow now.