CPU fan spinning slow, no display, power button not working

Check Power Connections: Make sure all power cables, including your main power cable and motherboard power connections, are securely fastened to your computer.

Make sure the CPU fan is connected securely to the motherboard; loose connections may result in slow fan spin. Clean out any debris preventing its proper function as well.

Reset CMOS (Complementary Metal-Oxide-Semiconductor): If the issue still persists, try resetting your CMOS by temporarily unplugging and reinserting the battery in order to reset any BIOS settings that could be contributing to it. This should allow any problems you might be experiencing with BIOS settings to go away.

Reduce Hardware Components to See if Problem Remains: Disconnect any unnecessary peripherals and RAM sticks until only one RAM stick remains installed in your computer, reboot and see if problem persists - if so, gradually add back components until one becomes problematic.

Check Your Graphics Card: If your PC features an independent graphics card, make sure that it fits securely within its slot and try reseating to ensure a good connection between all parts.

If none of these steps work to address your issue, it may be beneficial to consult a computer technician or bring the computer for professional diagnosis and repair.