To eject/unload the tape, you would use the library itself. Either from the front LCD or via the library's web interface.
Backup Exec unloads tape drives when the BE services are started, so assuming that you have already restarted the BE services, there is nothing more that can be done from within Backup Exec at this time.
Power cycling hte library should not hurt anything, but there are no assurances in life. If you cannot unload the tape using the library interface, you will have no choice but to power cycle the library and try again, either from the library interface or by restarting the BE services.
Depending upon the libray, it is also posible that you can reach into the library. Possibly to grab the tape and extract it, or possibly to nudge it in so that it loads properly in the drive. Sometimes drives have a (barely) visible eject button that may help unload the tape. Safety first though.