I am running out of cash

In rough order of how much it costs you:

1. Park or sell an idle aeroplane. An aircraft with no route is pure cost.
2. Close the worst route. Route economics ranks them by profit per aircraft-hour, which is the number that matters when aeroplanes are the scarce thing.
3. Raise fares on anything flying near full. Free money if you are spilling passengers.
4. Borrow. Interest runs about 13% a year, so this buys time rather than solving anything.