Funny !  Tim, none of us has taken a vow of fidelity to CS.
I fly my Cessna 172 for low sightseeing, I fly my Piper Arrow III - was my first more complex aircraft for low altitude cross country. This is about pleasure and satisfaction.

What you are experiencing is a partial loss of FMC - Vnav. Not a big deal, no need to be frustrated. It is a test of your knowledge !
Switch to MCP and descend , Lnav still works, you will get to destination and can still use Hold. Land the airplane with the ILS bugs, runway lights or visual, but land safely. A little challenge put a self satisfaction into your flight. Pilots train for the unexpected   ;D

Have safe flights !  Happy landings !  8-)

The FMC came with the B757 !

Raymond   Boeing or nothing
KFLL   South Florida