Vauxhall Zafira won't start, no ignition lights or power.

Started car up drove around 2 miles noticed the letter 'F' on the top right hand corner of dashboard display for radio & then noticed the headlights were on, but the light switch was in the off position as I hadn't put them on. I pulled in turned engine off grabbed some lunch as I was working & when I returned to car pressing unlock button on car key there was no flash of ndicator lights to show car had unlocked as I assumed I'd locked it. I put key in the ignition & nothing no dashboard lights or any power at all. Just nothing totally dead. I popped bonnet to check battery terminal's with the car key still in ignition with key turned into ignition on position, as if ready to start up and I noticed a kind of continuous buzzing sound coming from engine bay so turned ignition key to 'off' position then back to the 'on' position & the sound started again in a continuous sound but gradually getting louder. As it gets louder the headlights come on really dim and get brighter as the sound get louder. No central locking no power to anything at all. My battery is only 2 months old. Any ideas as to what it is would be great