Overheating.
Have you opened the case and cleaned out the dust recently? If not, please do so, ensuring all fans are dust free, use a soft cloth slightly dampened, then dry with a lint free dry cloth. Use a can of compressed air (available at all PC shops) to get to the bits you can't reach with a cloth. If it still refuses to boot up it probably means the CPU thermal paste needs renewing, this is a two minute basic job if you know what you are doing, there are videos on YouTube that show you how it's done. Thermal paste is also available at all PC shops.
If none of that works it could be a hard drive failure, which will require the fitting of a new drive and operating system.