Battery?

Did you leave any parking lights or interior lights switched on?

Have you checked the charging system?

Try this:

Charge the battery with a slow charger for at least 24hrs since its totally flat, then run the engine and check the voltage at the battery terminals, you should expect between 12.8v and 13.8 v. If you have the voltage in spec then turn on some loads with the engine running and observe the voltage, it should remain with in that range.  If the voltage is stable then turn the loads off and run the engine for a further 5 mins to stablise the voltage and to top up the battery from the load drain. Switch the engine off, wait five minutes and check the battery voltage, should be at least 12v. If its not then you may have a dead battery cell, typical 12v battery will have 6 cells with each cell at 2v, so 6 cells x 2v=12v.  You may have a defective battery.  

Hope this helps, let us know how you get on!

Hi had similar problem , battery wouldnt hold a charge and lights etc very dim had alternator checked all good put new battery on and made no difference had volts checked on engine idle and turned out to be the glow plug relay wasnt turning off even though the glow plug light would go out therefore only putting a high amount of charge into battery at high revs but around 2000 rpm it was charging about 11.7 instead of about 14 volts , making battery go flat , so check glow plug relay , good luck .