We have a carrier air conditioner and thermostat
The thermostat is set to cooling the same as it has always been but there is no cool air the fan is working
The thermostat has suddenly stopped working what could the problem be?

Could be a number of things. Do you have 24 volts to the thermostat? I would check that first. If no volts there check you transformer. Post back your results and we can go from there

Check the batteries in it. This can be done by pulling it off the wall and looking for a small piece that pulls off, usually containing 2 AA batteries.

Is the outside unit running?

The power is working on the thermostat but I think the temperature control is not. The fan is also working but there is no cool air.

Did you check like NorthernHeat ask? Is the outside unit running? To check the thermostat, jumper R & Y at the thermostat and see if unit blows cool air. If not then jumper R & Y at the air handler and see if it blows cool air. If it does then the thermostat is bad