2009 C5 won't start

[barrett90]

I've done the usual forum searches to try and find a definitive answer but not come up with anything that seems to be an answer that people agree upon for the exact issue.
I drove the car 10 days ago, no issue at all, parked up on the drive as ever. I have used another car since so haven't had to use it, but it's not the first time it has been stood for that length of time. WEnt out on sunday to empty the boot, no issue with locking or unlocking. WEnt back out on sunday evening to go out in it and it wouldn't start. AS others have described, the dash all lit up as normal but then just gave a click on turnover.
I tried to jump start the car but it wouldn't start. It did, on two occasions, half try and turnover, but we are talking for a fraction of a second. Aside from that, it just clicked.
I did notice a message flash up once on the sat nav screen which said 'internal vehicle fault, but it went off too quickly to read it all and didn't show up again.

Is it the battery not holding anywhere near enough charge to start up? Is it the key battery failed so the ignition won't allow the immobiliser to disengage? Has my car finally given up the ghost?

I can get my usual mechanic to come and look at it and get the car towed to his workshop but he isn't a vehicle electrics specialist and i've seen a number of posts about correct procedure for changing the battery and I don't want him to wipe anything from the car as i'm not certain where this mentioned 4 digit code is

[Stickyfinger]

First thing to try is a good battery ON the car. ....Even if low, still do the battery off /bsi reset procedure - as it is essential, C5x7's being particularity sensitive to it.
http://www.frenchcarforum.co.uk/forum/v ... 11&t=40407
Your battery should be a 75 or above.


and
Don't forget to follow the battery off/bsi reset procedure- as it is essential