Loose cables and hardware problems are possible reasons.
GNOME Documentation Project
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# My computer will not turn on

There are a number of reasons why your computer will not turn on. This topic gives a brief overview of some of the possible reasons.

# Computer not plugged in, empty battery, or loose cable

Make sure that the power cables of the computer are firmly plugged in and the power outlets are switched on. Make sure that the monitor is plugged in and switched on too. If you have a laptop, connect the charging cable (in case it has run out of battery). You may also want to check that the battery is correctly fitted in place (check the underside of the laptop) if it is removable.

# Problem with the computer hardware

A component of your computer may be broken or malfunctioning. If this is the case, you will need to get your computer repaired. Common faults include a broken power supply unit, incorrectly-fitted components (such as the memory or RAM) and a faulty motherboard.

# The computer beeps and then switches off

If the computer beeps several times when you turn it on and then turns off (or fails to start), it may be indicating that it has detected a problem. These beeps are sometimes referred to as beep codes, and the pattern of beeps is intended to tell you what the problem with the computer is. Different manufacturers use different beep codes, so you will have to consult the manual for your computer’s motherboard, or take your computer in for repairs.

# The computer fans are spinning but nothing is on the screen

The first thing to check is that your monitor is plugged in and turned on.

This problem could also be due to a hardware fault. The fans might turn on when you press the power button, but other essential parts of the computer might fail to turn on. In this case, take your computer in for repairs.