If your crank sensor was loose but undamaged it might well have generated a new instance of the P0335 fault. Re-seating the sensor might have cured that. However the stored code won't clear until a preset number of good starts is achieved. Might be six, can't remember. Or use a code reader to clear the fault from memory.
The V6 requires the crank signal to trigger the spark as well as the fuel. If there is a signal then the engine might start but run badly. But the engine should always crank. Since yours doesn't reliably crank you have some other fault which needs to be addressed first.
Check the battery voltage. Should be 12.5v or slightly higher at rest/no load. You don't want to see the battery voltage drop below 10v when cranking. Anything less then the starter solenoid might not work which would cause no turnover. If the numbers are any lower then you need to fully charge the battery, with a good charger. Or the battery might have a shorted cell and be scrap. And as suggested by Nick W above check the battery earth, starter connections and the engine earth.
Also the recent torrential rain probably hasn't helped. Did any water get into the car? Via the nearside scuttle maybe? Remove the pollen filter and shine a torch down. Is it wet or muddy?
Have you checked for oil and/or water in the plug wells?