Two things to consider :-
1) Physical problem, ie air vents blocked or dust in the case. Suggested remedies check work surface to ensure it's flat, clean and unobstructed, then take a can of compressed air to the vents and give them a blast.
2) Software problem, ie heavy load background application making the CPU work overtime, thus generating more heat and triggering the fan. Remedy Ctrl-Alt-Delete and check the running processes, shut down anything that's loading the CPU or looks unecessary.