2006 Vauxhall Astra complete power loss

Knackered alternator ?

Start by checking your battery it needs to be 12volts or above,if anything below that ,re-charge and check.then is it charging? ,you can check this with a multimeter on volt setting across battery terminals should read steady 12volts or above.and if you switch on load items such as head lights the charge rate should momentarily dip then rise back to original reading or above and this will tell you if the alternator is functioning.if there is no burnt smell from alternator then it,s probably the voltage rectifier that's faulty in fact mostly it is that.If none of the above then can only be wiring issue ,such as shorted cable ,or fuse blown.

If your alternator is showing 12 volts it's knackered .This explains better ,

http://www.todayifoundout.com/index.php/2010/01/how-to-test-a-car-alternator/