Case fans can die
The usual occurance is internal dust, therefore removing all internal dust can possibly help
In most cases it's a matter of replacing the fan
There is also another fan, close to where the power cord plugs in
This is actually the PowerSupply fan. Once this one dies, users tend to purchase another PowerSupply (as PowerSupplies are sealed units, and not meant to be opened)
You will need to work out if it's the case fan, or the PowerSupply fan, and then have it replaced