What engine and model? Any recent work on it? Has the engine been drowned recently? And not turning over on the starter at all? Nothing? No click? Measure battery voltage at rest and as you try to start and let us know. Check fluid levels in cells if appropriate. Check fuel pump and ECU fuses. When you turn the immobiliser off what does the light do? Check inertia switch connections and reset switch. You've got a multimeter. Would you like a wiring diagram? Separate starter button or on ignition switch? Check connections and if the latter wiggle the key as you try to start. Check relevant wires and connections: battery, ECU, starter, and relevant earths. Inspect, wiggle, disconnect, clean, reconnect. Jonathan