If an airline finds itself unexpectedly short of aircraft (e.g., due to aircraft going tech) then it basically has two choices:

- cancel flights, or
- use aircraft chartered from other companies

There are companies that do nothing but provide ad hoc charters to airlines that would otherwise have to cancel flights (for example, Titan here in the UK). I have travelled on chartered aircraft on several occasions, and the service has not always been as good as it would have been on the "normal" carrier for the route, but the view I've always taken is that I'd rather get home with Titan (or whoever) than spend time stuck at an airport.