Happened to me years ago on a relatively simple itinerary, 3 out of 4 flights cancelled. Was a weekend business hop back home to LA from Oregon where I was studying. EUG-SFO-BUR-SFO-EUG. The first segment was cancelled and I was only told so only after arriving at EUG at the counter. Seems the CR2 leaving SFO went MX. Counter CS agent got me booked last minute onto an earlier flight just preparing to leave then.

Next segment SFO-BUR was the only one not cancelled. On the return received an automated phone call from UA (this was before smartphones and apps) notifying me that my BUR-SFO flight was now cancelled. Called premiere line, waiting forever (it was April and there as a big blizzard in DEN so the whole system was stressed) and got last minute booked onto a mainline LAX-SFO flight to still make my original connection to EUG. After arriving in SFO and just at boarding time for the flight to EUG the GA announced that they were cancelling due to having no crew for the flight. Somehow they didn't know that there was no crew until right when we were schedule to board, still never figured that one out. Most were told they would have to wait until the next morning when a charter bus would be provided to EUG, all were given hotel and meal vouchers. Myself and a few other savy passengers pushed for something better and got booked onto AS via SEA the next morning. Longer traveling distance but we still arrived in EUG way before the bus was set to arrive. Also still got them to give me MP miles for the AS segments.

After the whole ordeal was over I did call CS to complain about 3/4 of my whole itinerary having been cancelled and did receive a voucher, $200 I think, though I can't remember for sure.