There is usually diagnostic LEDs on the Logic Board of the iMac Logic Boards which provides another method to determine where a fault lies (or at least where to look) especially if there is no video output. It is usually fairly easy to locate the Apple service guide for your iMac.
What does "the Apple Diagnostics and my mac refuses to go there" mean? Details are very important since we cannot see what happens including exact error messages.
Plus you have no idea if the parts you purchased are even any good. Even when purchasing parts from a vendor with a stellar reputation there is always a possibility of receiving a defective part. Plus once in a long while one bad part may damage a second part, so when you install a single new part the other bad part may sometimes damage the new good part. Luckily this situation is rare, but it is always something that must be considered. Normally a computer will only have one defective part when there is a problem unless accidental damage or a lightning storm or electrical surge were involved.
FYI, the 27" 2011 iMac has a known GPU failure so that just adds yet another complication to the mix. The diagnostic LEDs on the Logic Board will be crucial in trying to diagnose the problem.
