When the ECU battery is plugged in, regardless of the receiver switch position or whether or not the transmitter is on, the ECU will continuously drain the battery, eventually discharging it.
The JetCat manual is very specific about this, suggesting unplugging the battery at the end of each flying session, if you use Lithium batteries to prevent overdischarging. They do not recommend using lithium ion cells. (high impedance)

If the battery had been drained sufficiently, the reason for the powerfail warning is obvious.
I suggest you completely charge your LiPo batttery and give it a go at full charge after conducting a full check and completing the repairs.

Another sugestion for you is to NEVER blow a fire extinguisher up the tailpipe...always discharge it down the inlet, even if the bypass cover is installed.
Reason: You don't need any burning fuel in your (or anyone elses) face!