My computer keeps switching itself off and won't swtich back on again? What can be the problem?

1) Bad Power Supply. 2) Bad Radial Aluminum Electrolytic Capacitors on the motherboard,http://capacitorlab.com/visible-failures... 3) Computer is dirty inside. Processor has been overheating, and turned off. So many times, and it won't turn on again (BIOS rules this), until the problem is corrected. Power Supply can be checked with multimeter. ($5 to $12) Check the 3 main voltages coming out of the Power Supply. [ 3.3 Volts DC, 5 Volts DC, and 12 Volts DC. In comparison two D cell flashlight batteries store 3 Volts DC. Dangerous voltages are contained in the metal case of the Power Supply ] Or use a KNOWN to be good, Compatible power supply borrowed from another working, (Unused?), computer, for a test unit. Post back in a Comment if you would like guidance, and post the desktop computer manufacturer name, (LG?), plus the model number. (Back of computer next to the Windows product key, or up on the side of the computer tower. Model Number. MDL ) 1) Bad Power Supply. 2) Bad Radial Aluminum Electrolytic Capacitors on the motherboard, http://capacitorlab.com/visible-failures/index.htm 3) Computer is dirty inside. Processor has been overheating, and turned off. So many times, and it won't turn on again (BIOS rules this), until the problem is corrected. Power Supply can be checked with multimeter. ($5 to $12) Check the 3 main voltages coming out of the Power Supply. [ 3.3 Volts DC, 5 Volts DC, and 12 Volts DC. In comparison two D cell flashlight batteries store 3 Volts DC. Dangerous voltages are contained in the metal case of the Power Supply ] Or use a KNOWN to be good, Compatible power supply borrowed from another working, (Unused?), computer, for a test unit. Post back in a Comment if you would like guidance, and post the desktop computer manufacturer name, (LG?), plus the model number. (Back of computer next to the Windows product key, or up on the side of the computer tower. Model Number. MDL )