My central air unit worked fine last night. In the morning, the unit works but the fan is now very slow. Cold air comes out of the vents but no force and not cooling the house. Why would the fan just all of a sudden run slow?

There are better people than me to answer this question, but my guess is that your evaporator has frozen up. This can happen when you are low on freon.

Try leaving the air conditioning off for 30 minutes or so--to allow the ice in the evaporator to melt. Then, when you turn it back on, see if the airflow is higher. If so, you should probably get your system serviced. Actually, you'll probably need service no matter what, but at least you'll know what to expect.

Also, make sure your filter is clean. This can explain low airflow, but not the sudden change in airflow.

Good luck,
WallyH