1985 Buick Electra no heat

I have a 1985 Buick electra 380 I had it parked for two months and when I started driving it again the heat did not work and it was over heating so I put a new thermastat in it and got a new radiator cap and I still don't have heat and it's still overheating and the back pressure is making the anti freeze come out of the radiator cap or around it some where what could it be.