Hi

It’s possible, but unlikely for the logic board to be damaged. Logic boards and memory modules are electrostatic sensitive devices. Touching them can potentially create small electrostatic discharges that would damage them.

Most likely:

1. the new memory modules are not compatible or faulty. usually the system will beep when you try to power it on and there is a memory issue. But not always. I myself have a laptop which had a faulty memory module and simply refused to turn on. No beep, no nothing. Replaced the module and all was fine.
2. some of the connectors are not properly seated. Because the entire logic board needs to be removed in order to access the RAM slots, there are multiple connectors which need to be disconnected and then connected back.

I would try the following:

1. open the iMac again and carefully check that all connectors are plugged back correctly.
2. if this does not work, put in the old memory modules and check

Good luck!

Bogdan