Our lennox furnace fan runs constantly, but our 2 ton lennox air conditioner is not running,and the temperature in the house is in the high 70's Could this be a compressor or freon problem.

Could be a lot of things
" " " breaker problem
" breaker problem
" " low voltage wiring problem
" " " bad contactor
" " " bad capicitor
" bad contactor
" " bad fan motor ( and compressor shut down from being overheated)
" " low on refrigerant ( and low pressure switch won`t allow compressor to start

If it was a bad compressor the fan should be running

Check these things out and let us know

We need some more info to make an informed guess.