how can i fix my central locking on my 1999 vectra?

My central locking suddenly stopped working using the buttons on the key. I changed the battery in the key but it still did not work. I locked my vectra manually using the key in the drivers door all the doors locked and unlocked but i cant turn off the alarm every time i open any of the doors or tailgate the alarm goes off even when the car is unlocked. I have tried reprogramming the key by sitting in the car turning on the ignition pressing the lock button but it will not program. when you push and hold the lock button the red light flashes but nothing happens. please can anybody help me.
Thanks
Andy