Fever can be indicator that the organism is fighting against something foreign that attacked it, like viruses, or bacteria, or some other, more serious sickness like cancer. In most cases it is harmless, but if it persists and is too high, it might cause some complications.
If a person has been exposed to a heat source (sun, a room that was too hot, etc.), and/or was overdressed at the time, fever can be result of a heat stroke, in which case immediate medical attention is mandatory.
Note: As this is a health-related problem, we suggest that you consult with your doctor first.