A few things may have gone wrong if you cannot find one or your disk drives in My Computer. But let's assume you have already verified that the drive is present in your hardware configuration. If this is the case, the most likely explanation to the disappearance of your drive is that it is not properly formatted and it is not recognized by Windows.
Here is the solution from Microsoft:
Use Logical Disk Manager to make changes to logical drives when you have logical drives using both NTFS and the FAT file system. Fdisk is not supported for use in this configuration.
To open Disk Manager, perform the following steps:
Click Start, right-click My Computer, and then click Manage.
In the Computer Management MMC window, click Disk Management in the tree under Storage.
The Logical Disk Manager will display, showing all available disks and their partition information.
Select the drive that is not partition by right clicking on the icon (usually it will be labelled disk1 since disk 0 is your primary disk)
You can now delete the current partition, create a partition and format the drive. Once you format it you will be able to use it.