If your Opel Vectra is not starting, there could be several possible causes. Start by checking the battery for proper charge and connections. Ensure that the fuel tank has an adequate amount of fuel. Check the ignition system for any faults such as a faulty ignition switch or spark plugs. Inspect the electrical system for any blown fuses or loose connections. If you are unable to identify the issue, it is recommended to consult the Opel Vectra automotive repair manual or seek professional assistance to diagnose and resolve the problem.