As Red said check nothing is on, disconect the main lead and see if there is much sparking (when engine off). If there is something is draining it.
I have known new batteries fail, and also batteries seem to fail very quickly these days, one day good next dead,,,,best thing to do is have it tested, its a quick and easy thing to do in any garage or motor factors. I also like to fit a cut off switch as this stops any drain,,,but is not good with alarms..lol
Good luck
Phil