As you have been away for a month it is probably that the battery has just died as the alarm has been on.
It is that time of year when you find out just how good your battery is. I know the RAC will come out to you and fit a new one for around £90 even if you are not a member. I know you don't want to spend out if your existing battery is ok, but if it is old may be worth doing and you can kill two birds with one stone sort to speak.