It’s possible it may be a hardware issue like something overheated or you have issues with your power supply. Those are “fun” to replace. IT may also be virus related. Try booting the computer in safe mode and run a good antivirus program like Malwarebytes or MWAV, Double check all you connections and make sure the memory is facing the right way. A friend of mine put one memory chip facing the wrong way and his system would not boot.
Another possibility that some may forget. Depending on the age of your computer, if over 5-7 years, a weak CMOS battery may prevent your computer from booting up.