immobiliser won't allow me to open car when I click 'unlock' on fobr?

Hi I can't get my car key fob to unlock car - it just makes a high pitched bleep. So I open the door manually & then I turn the key in the ignition & it won't start the engine. Does any one know what I need to do, please? I have changed the batteries in the fob. Do I need to reprogramme? Also I have checked the main car battery with a volt reader & that is fine