If your Astra(F or G) or Vectra (B) remote stops working its normally one of two things wrong. Battery low or its gone out of sync with the car, worse case of course it couold just be buggered but thats rare as they are pretty robust.

First try to reset the remote. To do this put the key in the ignitionj and turn to position 1. Then press and hold the open door button, the door locks should “clunk”. Thats the remote reset, if not try a new battery and give it another go.

If agian it dosnt work sadly you may need a new remote. At best a vist to your local Vauxhall dealer who may be able to re programme the remote using Vaxhall Tech 2 Diagnostics.