my canon xti auto focus wont work with my sigma lens?

I have a cano xti with a 28-70mm F2.8-4 DG sigma lens. It work fine for a while but then it gave error 99, I cleaned contacts with rubber eraser and it didnt come up, but now when the lens is on auto focus i get a blinking letter going from af to ma and it wont stop and wont let me shoot anything so i have to put the lens to manual in order to work with so i have to literally focus on my on. What it can be can somebody help me.