It's happened to me on more than one occassion.
You drive a few dozen miles to the gig.
you set-up and ready for sound-check.
The red light glows "on" and then zipp/crackle/phart, your amp flatlines on you.
The first occassion I was lucky - we had a gig with a great house system and sound team. They ran me direct and pulled out an extra bin from storage to serve as my monitor.
I had to admit the sound the team produced for me was a vast improvement over the dead rig.
Another occassion, we played a small bar-gig in the city.
My amp speaker blew. I had no Di with me and the PA system was pretty much for vocals and not up to snuff.
We almost had to cancel. I wound up plugging into the gutarists twin reverb - it was weird, and tinny, but it got the job done.
Since then - I carry two Di's with me (at least in the current band that travels with PA) and in a pinch if Di is not a solution I have a list of phone numbers of bass players and rental companies for a last ditch effort..
What is your bass-amp gig-rig fail-safe?