(This kind of blockage in the air path can kill a PC)
To try to fix the problem on your own, get a can air-duster. (Do not use an air compressor, the pressure is too great and will break off fragile fins inside the PC)
In the air fins, do short blasts of air and you will see dust shoot out. Keep doing short blasts until the dust stops coming out. Do not do extended blasts of air, it can cause the PC fan to spin too fast and damage it.
Make sure to move up and down the fins.
If the problem continues, contact us and we will be glad to diagnose if it is a fan failure, or more expensive motherboard failure, which could mean it's just time for a new PC. But typically, cleaning or replacing the inexpensive fans, will solve this problem.