A low battery can cause failure to start as the electronics require at least 10 volts to work properly.
After a flat battery you can get indications of full communications to all units and no fault codes - just no start. Martin Robin Swinden shows a way out of this lockout using PP2000.
Martin says - The only way I have overcome it is to run PP2000; and enter into "new vehicle preparation" menu.
It asks all sorts of questions you can't answer but by lies and intuition noting down the answers, getting rejected then trying the other options you will finally get to the bit where it says "Program whole vehicle" - you will need the pass card with the car immobiliser code. Enter and by a miracle not more than 30 secs later it should say "turn key to start" - and it does.
The first time Martin did this was after sending his ECU and BSI units to test centres to be checked over. Still no start. With nothing to lose Martin went for it with PP2000, and was successful. He reckons the security chain to be KEY - BSI - ECU and the loss of continuity seems to be between the BSI and the Engine ECU which have stopped talking to each other (plus the brake light switch on autos).