I have a 2003 Buick Rendezvous

yesterday I drove my buick just fine.  I turned on the air conditioner and drove 7 miles.  I stoped at a place bought some gas and food at the store, got in the truck to head home . i drove 2 miles and it died. I tried to start it but it wouldn’t start. Today my mechaninc fixed it and charged me $150 dollars I drove it home,  then to my daughters ball game and did fine. We left the ball game and again went 2 miles and it died. Iit was all I could do to turn it in to someones driveway but within 15min it started up so I drove fast to get home but again within 3miles it totally died. What is wrong with it?