rover 25?

Hi drove my car to b&q went inside for ten mins, came out and my car would not start. The lights on my dash board all come up but nothing not a sound. I had my batterie changed at a garage a month before could this bethe immobiliser and if so how to a reset it. It started up after someone gave me push drove 5 mile back home ok, Then turned the engine off and then back on and it was dead again.