Try this, Turn off the computer, remove the power cable connected to the power supply, reset the CMOS or BIOS( Refer to the motherboard's user manual ), If you don't have the manual or if it doesn't work, remove the CMOS battery for 10 - 15 seconds, then install the battery again. Clean the RAM too, the procedure is shown on the image below.

If you have a graphics card (Video Card) installed, refresh the connection (Remove and Install). Then turn on the computer. Hopefully this will solve your problem.
Note: Keep the Components of the Computer Dust-free (ex. Motherboard, CPU Heatsink and Fan, RAM, etc.) for it may cause malfunctions on the Computer,
Please reply if this solution was helpful. Thanks.