More information needed.
Update your post to include full system hardware specs and OS information.
Include PSU: make, model, wattage, age, condition (original to build, new, refurbished, used)? History of heavy gaming use?
Disk drive(s): make, model, capacity, how full?
Try to identify the specific fan that keeps turning on and off.
Any lit LEDs on the motherboard?
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The starting point is for you to go to the motherboard's manufacturer's website and find the applicable User Guide/Manual for the motherboard.
Use the manual to: 1) determine what the single beep and described beep pattern may mean. Check what any lit (or unlit) LEDs may be indicating, and 3) Troubleshoot the motherboard carefully going step by step and checking everything.
You should also look for specific component manuals: GPU, Case, PSU, etc. as applicable.