If you are referring to the blower motor there are two common issues with them. First is the blower motor resistor. It will typically take out the higher fan speeds like while the lower one or two speeds will work fine. If you put it on high speed and it stops all together, its likely the blower motor resistor.
The other possibly culprit is the ignition switch. All the accessories run through the ignition switch and as a result of the load, carbon builds up on the contacts and can cause the blower motor to quit working. However, you usually find other accessories quit working before hand, like the radio or the thing wont start, etc.
If all the fuses are good and you are not having any issues with other accessories I would start with the blower motor resistor since it is the more likely culprit.
