Hardware support plzzz

Hello , so I costumed built my water coolant pc , everything was running great . Then one day I decided to upgrade my graphics card to a better one so that I can game . I had on there working GeForce 8800 GTX and the graphics card I bought used from somebody on offerup to try to upgrade it was GEFORCE GTX 550TI . So I swapped the cards when I went to turn in my computer everything powered on normally from the looks of it , my fans , water cooled system , led lights etc etc , then after awhile all it kept saying on screen is no signal . After days of trying to go through process of elimination trying to figure out why all I got was just no signal I couldn’t figure it out . 1st process .. was checking the dvi , mini hd to hd cable , pc to dvi converter cable , and nothing still no signal . Then I checked my memory sticks one by one on my ASUS P5Q PRO motherboard . Memory installed on there working is CORSAIR XMS2 DDR2 800MHZ 1024MB . Nothing still . Then I swapped out the cmos battery , and then tried moving the 550TI to the below PCIE 2nd slot . Still no signal . so to my frustration I put my original graphics card back in . But now I still get NO SIGNAL . So I went to my garage pulled out other hardware to get another computer going just to check if anything got burned out or if I got scammed on the graphics card . Got the second computer going ... basic dell tower with basic stuff . I put in the graphics card I just bought from the person on offerup and sure enough it is working . Tried my original working card I had in my custom built and it works as well . so I just figured out must of maybe been my power supply that maybe shorted because it has a burnt lil smell inside of it . prior to checking the power supply I put my original card back in one more time . When I did this my custom built just sounded and looked like it shorted out . but nothing looked or smelled out of the ordinary . Disconnected the dc cord to the power supply and it started back up but I noticed that my LED light was flickering & my 3 prong fan connections to some of my fans didn’t start up unless I connected them by the molex connectors then they all started back up again . back to the power supply issue I swapped out my original CORSAIR HX1000W power supply to another power supply “DELL D750E-00 750W power supply . after that still have the same issue . NO SIGNAL . My question is do you guys think I burned the motherboard out or something in it or the cpu chip ? Please help me , as I’m going crazy with this for 2 weeks now . Thank you so much .... please advise !

The power supply was the one smelled burnt . Already replaced it and still same thing

Just don’t know how it got fried . And if it got fried. Shouldn’t it technically not start up or something ? Because everything turns on . Water coolant pump , led lights , ... hows that possible then . ? Thanks for ur help