Please help.

Have a 306 XSi 3 dr N-reg,with the keypad immobiliser. When you turn the
key, both lights come on.  Tried a few of the tips on here and the
disconnecting and reconnecting worked briefly, but now cannot get the car
started at all.  The engine turns over, so don't think it's mechanical.
You can type in numbers, but they have no effect, although if you type in 3
times it locks the system.

Have read that if we get it started then we can disconnect it.  Just need
to get it started.

Any advice?