Assuming your coolant level is good, you could have a multitude of issues. Plugged radiator won't let coolant flow, T-stat won't open, plugged water jacket in block, water pump fins have corroded away. The last one is highly unlikely, but still possible. I would start with a good flush and check to see if the T-stat is working. Put it in a pot of boiling water.
Also check to see if the front of the radiator is blocked with depris, leafs, grass, etc. . .