Car not starting possible immobiliser problem

[Samantha]

Last week when I went to start my Astra 1.6 55 plate it wouldn't start and the lights were going mad.I got a jump start but my headlights kept flickering and the power steering kept going heavy.I got a new battery fitted which fixed the problem.Yesterday I noticed my amp wasn't working so my brother had a look and there was a loose wire and a fuse in the boot had blown so he fixed this but then my car wouldn't start.Someone gave me a jump start but it didn't start so tried to bump start it and again it didn't start so I phoned the aa.He checked the battery,alternator and starter motor n everything seemed fine.He connected the diagnostic machine n showed no problems so had to get towed home.The orange car light with the spanner on the dash is staying lit but there's absolutely nothing coming from the engine.I tried my spare key and the pedal test but it didn't show up any errors.Could it be the immobiliser if so why would it stop working?