Servicing/fixing an Ultegra bar end shifter
There are pawls inside that allow for the clicking of one gear to the next.  After years of service it is quite common for those little pawls to shear from their body and become lodged in between the shift space of two gears.

Even if you free the pawl the shifter will only work in friction mode as the pall will no longer be there to click anymore.

8-speed ultegra should be able to found readily and cheaply.