Try checking compmgmt.msc - Diskmanagement and see if the partition still exists and if the computer can pick it up. If it is listed, but there is no drive letter, right click it and assign one. Sometimes USB drives conflict with one another. If it is in My Computer, but still not accessible try doing a checkdisk. Load up the command prompt and type in chkdsk (drive letter): /r... so if it was d it would be chkdsk d: /r

Hope this helps