2014 5k Retina iMac will not turn on after ram upgrade.
I have a late 2014 iMac 5k that will not turn on after I upgrade its ram from 8gbs to 32gbs and when I swap the old ram back in it STILL will not turn on.
Earlier Mac models
Sounds like maybe you are not fully seating the factory RAM modules in the slots.
1) Reinstall the original Apple modules in the same slots that they were originally in and test.
When installing, push firmly on each module with two fingers to ensure that each is firmly seated.
see > Install memory in an iMac - Apple Support
2) Mac are very RAM picky and will not work with cheap aftermarket RAM or RAM that is made for a PC.
Before you try re-installing the aftermarket RAM modules, please share a web link for the aftermarket RAM modules.