See if the monitor is on. If not, you will hear a beep after the fan is turned on and on for a few seconds. If there is this sound, it means there is nothing wrong with the hardware, and is your computer an integrated graphics card or a discrete graphics card? If it is a discrete graphics card, I suggest you unplug the graphics card and plug it in again to see if the interfaces are all connected. If the graphics card is ok, then your motherboard is broken.