Try turning the computer off (not just hibernate or standby) and power down and disconnect the external drive.
Reboot the computer and wait for Windows to load and logon.
Reconnect the drive, switch on and wait for any prompts or indications XP offers. You may see activity on the taskbar by the clock. If you are prompted for a driver disk, use the one that came with the drive.
Check your My Computer again. If still no joy, try rebooting the machine with the drive on and connected.
Good Luck!