Is the starter motor just clicking when you try to start the car?
First thing to check is the battery voltage with a multimeter when your car refuses to start. If the voltage has dropped below 12v then something is draining the battery. If you find the voltage is fine then that would point towards the starter motor being faulty.
If you find you have a parasitic drain, here's a good guide on how to find it:
https://testmeterpro.com/parasitic-d...-a-multimeter/
If you find you're drawing to many amps, it's a matter of removing 1 fuse at a time until you see a sudden drop in amps.
When I tested our TF as I thought we had a parasitic drain on that (turned out to just be the battery) we found a very small amp draw of something like 5 milliamps from memory.
Do you have the highline satnav system with the satnav drive in the boot on the right hand side? If so this is known to keep on running after switching the car off which can drain the battery.