We have a lennox ac unit less than 5 yrs old. The furnace will turn on, and it feels like cooler air coming out of the vents, but when you check actual unit outside, the fan is not running. The top gets very hot when its trying to run, and its making a humming noise, but no fan movement. Any ideas?

Take a stick,try to force spin the fan, does it start up? Is it a little hard to spin with the stick?is the compressor running?

The fan started running when I moved with screwdriver, and the fans inside are now blowing, but its not really sending out any cool air.

Also now the fan won't stay running

When you turn it with screwdriver/stick does it move freely or does it have some resistance?

Yes moves freely

Your outside unit is a compressor and a fan drawing air across the coils. Sounds like your compressor may be running but without that fan its going to fry. You may have a bad capacitor in the fan motor which is not very expensive or difficult to fix or you could just buy a new fan motor. I'd disconnect and remove the fan and try bench testing it before doing much else.

How would you bench test? An hour ago the upstairs unit (which is the one with the problem) tripped the breakers for the downstairs unit, and now both are not working.

Bench testing would involve having the motor only on a work bench and apply direct 120 vac to the hot and neutral leads from the motor. If it still won't spin replace the capacitor, still no spin buy a new motor. You were using outside/inside to describe your units, lets stick with that. This is a Lenoox furnace with AC, right? The outside is your compressor and fan for cooling coils. Inside is your furnace for heat, A coil and fan for cooling. You should not let one unit run if the other unit is in failure. The power appears to be off but double check that nothing is still hot. Don't want you opening the outside cage with power still on anywhere

We have a lennox ac unit less than 5 yrs old. The furnace will turn on, and it feels like cooler air coming out of the vents, but when you check actual unit outside, the fan is not running. the top gets very hot when its trying to run, and its making a humming noise, but no fan movement. Any ideas?

At less than 5 years old,the motor should be under warranty,check into it but keep in mind and ask what the labor fees might be, something to think about also doing it yourself could/might void the warranty