You need to get to the underside of the tape drive itself, there is usually a manual tape release mechanism built in however it usually requires the user to wind the tape off off the spools until it is fully disengaged from the spool and then to unload the tape there are usually magnetic servos and lever arms that need to be moved to release the tape itself, without being able to see the drive myself I could not begin to explain further as each manufacturer's mechanism is different, however the tape release looks like a recessed screw on the underside.

it is possible that if you manage to rewind the tape using the manual release, that reconnecting the drive may allow the drive to eject the tape using the eject button on the front panel