Can you give us some brief specs, or make & model?
Couple things to check out (with computer unplugged)
1> dust - you want to check the CPU & GPU heatsinks to see if they have any dust build up, if so, the dust needs to be cleaned off. If possible vaccum the dust out (compressed air just blows dust from one spot to another), but be sure to stop the blades of any fans from spinning while vaccuming. The suction created by the vaccum can cause the blades to spin far faster than their rated specs & burn the bearings up. Also check for any intake air filters, if your case is equiped with these, they need to be cleaned at least once a year, more often if you have pets, or live in a dusty environment.
2> case airflow - check out where any intake & exhaust fans are located, then look for obstructions between them. Ideally, you want front low intake fans & rear high exhaust fans. If possible, move any obstructions, i.e. tuck loose wires behind drive cages, use zip ties if needed. Depending on motherboard & case, you may even be able to move a video card (or other add-in cards) to a lower position for better airflow.
3> dead fans - any dead fans are a bad thing, but especially if they are on the CPU or video card. To check, just try to spin them by hand. If there's any resistance, or they stop spinning very quickly (instead of a gradual stop), they probably need to be replaced.
With computer running, check your CPU & GPU temps at idle, then under load. If there's a significant increase in temps between idle & load, you may need to re-apply thermal paste, or your heat sink may just not be up to the task. The later can especially be true if using a stock heatsink. On the CPU side of things, the CoolerMaster Hyper 212 (several variants available) is an affordable & very effective upgrade to ANY stock heatsink & has the option of up to two 120mm fans for improved airflow at lower noise levels.Hyper 212 Plus (currently on sale)Hyper 212 EVO