|" I think it is stuck in sleep mode "|
The computer system CANNOT stay in "sleep" mode.
You can have problems with Windows not recovering from Standby or Hibernate modes, if you installed Windows from scratch and did NOT load the main chipset drivers for the mboard after Setup was finished, but in any case, if you hold the power button in or down until the computer shuts off, the system cannot be in "sleep" mode when you start up the computer again.
If the computer will not start up normally in that situation, something else is wrong.
If this is a desktop computer.......
The CPU fan must spin, and it must be connected to the correct 3 or 4 pin CPU
fan header, otherwise most mboards will shut off in a very short time to prevent the CPU from burning out.
However, the computer does NOT restart automatically when it does that.
Failing power supplies are common and can cause your symptoms.
If you can borrow a power supply from another working computer, try connecting that.
If you have a video card installed in a mboard slot....
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD ! )
If that info is not in the ad for the video card, you can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12 v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements. Some power supplies have two or more +12 v ratings - in that case, add those ratings to determine the total +12 v current capacity, OR, if the max total amperage or watts at +12 v is stated, go by that instead, whichever is less, (max watts at +12 v divided by 12 = the max amperage at 12 v).
An inadequate power supply may work at first, but it will be damaged eventually from it being constantly overloaded.