how do i mend my citroen xantia 1.9 td r reg?

my car has been lying in the drive way for the best part of a year with my friend comming over to start it up every 2 weeks or so while ive been overseas the problem is that the abs warning light will not turn off when the car is running i dont think that the abs is working either i have looked for the sensors on the calipers but there appears to be none on the back and front also the back brakes are recording little or no effort when pedal is depressed any help would be much apprieciated