It could be a hardware problem (as suggested by the event 15). I suggest you do all of the following:
* Check whether the library is visible in device manager and check for error codes on the hardware. If there's nothing in device manager or there are error codes, then check the hardware, cables, terminator etc.
* Check/Upgrade firmware of tape library and tape drives
* Install the latest device drivers.
http://seer.entsupport.symantec.com/docs/300325.htm (for 32 bit)
* Reboot if required.
* In Backup Exec, right-click the tape drive(s)and uncheck Enable. Right-click again and choose Delete.
* In Backup Exec, right-click the robotic library and uncheck Enable. Right-click again and choose Delete.
* Close the GUI
* Stop Backup Exec services.
* Run program files\symantec\backup exec\tapeinst.exe. Choose to "Use VERITAS tape drivers for all devices". Also make sure to check "Delete entries for tape devices that are not available". run through the wizard.
* Start the services
* Start the GUI - try again.
If all this still fails, you most likely have a hardware problem. Call the hardware vendor. Or check change cables/terminator/scsi card.