Peugeot 406 TD Won't start?

My 406 S reg was running fine in the morning went out about 2 hours later and it won't start. All ignition lights on. Got no button on key fob to turn alarm on and off so just use key to open doors. The drivers side door won't open it just spins in the lock so been opening central locking by passenger side. Now it just turns over and won't fire. Have been told its immobilised itself, heater plugs gone or cellinoid seized. Has no other means of putting alarm on apart from key and that does'nt work. Been using car like this for approx. 4 months now all of a sudden its done this. Anybody any idea how I can fix it Please!