Megane 1.5dci 'low battery' and won't start

[Simon_16v]

Last week my partner left her car windows open slightly when we had all that rain.
Since leaving them open, we've tried starting the car and the radiator fan runs flat out and the dash displays 'low battery'.

I have tried jump leads in case she had left anything running but that has no effect. A dehumidifier has been running in the car for 24 hours or so and has pulled quite a lot of water out so far.

Is there anything obvious I should be checking?

Thanks

 

[JamesBryan]

Water has possibly got into the fuse box area on the passenger side dash?