Did you check for bent pins on your motherboard socket?  A bent pin can be one cause of an endless boot loop.  It's easy to bend the pins by accident without knowing it, if you apply pressure to the IHS when you remove old thermal paste (e.g., if you wiped down on the CPU to remove the paste while the CPU was installed on the motherboard), install the CPU down at an angle, rock the heatsink back and forth sideways when removing it from the top of the CPU, etc.  If so, you can try straightening any bent pins with a sewing needle.   Also, keeping in mind to always discharge flea power before opening up the PC by pushing and holding down the power button several times after unplugging it from the wall so you don't short anything on the motherboard.  You could also try reseating the power cables on the motherboard. 
Hello! I'm not sure where to even start but here is the best I can to explain my situation. A while back (about 3 months ago), I have upgraded my GPU, from a GTX 745, to a GIGABYTE GTX 1650 OC Low Profile edition, alongside my old 180w PSU, to a 330w PSU. No problem there. I have recently purchased a used, but working i7-4790k in order to upgrade my old i3-4150. My PC was working just fine right before I decided to upgrade my CPU (alongside the latest BIOS as well). So I opened up my PC, gently pulled out the cooling fan, and then the CPU itself, while cleaning both of its old stock thermal paste with alcohol. I then put the i7 on there (with the arrow pointing to the right angle), closed and tightened the lid, and put the new thermal paste on it in a dot at the center. Then, I put back the cooling fan (already cleaned from the old thermal paste) and applied it to the i7, making sure the screws were tightened up as well.    I then boot up the PC and the lights flicker white for less than a second before not turning back on again, while the cooling fan and PC itself is still up and running. It won't even display anything on the screen and would simply act like no HDMI is inserted at all. I then decided to turn it off, disconnect it, and try resetting the BIOS with the little gold coin it has inside by sticking it out and sticking it back in after 5 minutes and after turning it on, it still stays the same with no way to boot into the BIOS at all.  So I decide to put back the old i3 CPU i had, reapply the thermal paste, put it back together, and once I turn it on, the lights turn blue like normal, but then the entire PC turns back off all of a sudden, turns back on, and repeats this process over and over again, until I disconnect it completely. I had this turning on and off problem over a year ago, until I upgraded my BIOS and reset it multiple times, which I tried resetting it now, but it still comes up with the same results. I'm not even sure what to do anymore and I'm afraid my PC isn't usable at this point after a failed attempt to upgrade to an i7-4790k, which from what I've gathered, the x51 R2 is compatible with the CPU, as long as the BIOS is up to date. Any other potential alternative solutions would be greatly appreciated! If I may have missed any more details, I'll be sure to provide them for you, if I can.  
Alienware x51 R2 CPU Upgrade Problem