This happened to me when I made a secondary flight plan (KLAX/KSAN), selected SEC F-PLN -> KSAN -> STAR -> LOC RWY 27 w/ LYNDI transition and no STAR. As soon as I hit *INSERT, the FMC went blank, and all the button stopped responding.
The primary F-PLN was KLAX/KLAX, with only RW25R as the departure waypoint and no other nav points. I was just flying around using heading select and LVL/CH while testing some things, and decided to ad-hoc land at KSAN.