After reading all the symptoms, I was almost certain it was your Graphics Processing Unit but after reading your specs and noticing that a GPU is missing I’m going to go with a Power Supply Unit failure. Usually a blown capacitor. It’s repairable but I wouldn’t advise it. Always invest on your PSU, it protects your computer which is more expensive than the PSU. Also, capacitors 99% of the time don’t blow up by themselves. You might want to install a UPS or straight up a Voltage Regulator to prevent future accidents.