1) check it is plugged into the power first
2) Is there a fan in the power supply? If so then put the side back on. These things are designed to work that way. It may be that the case fan is not needed.
If the PSU fan works then download something like speed fan from here click here
to check the temps in the box. Look around the web to find what suitable temps are. If the ones reported in your machine are OK then don't worry. My case has three case fans and none work, they are not always necessary.