Usually there are at least TWO fans in a computer.
One is on the power supply, and one is on the CPU.
There are also case fans, but you might not have those.
It is very abnormal not to hear a fan running (But you might just have good silent fans, you never know)
You should check which fan is running, and which is not.
It would be very helpful if you could unplug everything out of your computer, pull the computer out of where you have it, pull the side off, and plug the power cord in and turn it on. (Don't worry this is perfectly safe)
See which, if any fans are running, because if they are not running it is very damaging to your computer.
Here is some troubleshooting.
If the CPU fan is not working, check to make sure it's plugged in (Stupid I know, but always check)
If that fails, just go for a new fan, it's like $20.
Another thing, not all computers have a fan directly ABOVE the CPU.
My computer has a fan that comes from the front, and has a tube that goes to the CPU.
If the Power Supply fan is not working, this is also bad for your computer, and don't fix the fan, try to get a new power supply.
When you do get your power supply, try not to get a cheap one, they are often overlooked in computers, but they are what brings the system together, if that is cheap, everything else will work like it's cheap, no matter how good the system.