Hello quistan,   Sorry to read that. It doesn't look good for me. What I think is that the server sufferend any kind of damage (electrical or physical) during the intervention. Did you use a ESD wrist strap? ESD is a major concern when handling components, especially expansion cards and system boards. Very slight charges can damage circuits in ways that may not be obvious, such as intermittent problems or a shortened product lifespan.   If this was the cause, you can try to clean the flea power. In order to do that follow this steps: How do I Reset and Drain Power of my Dell PowerEdge Server?    By the way, a single LED indicator (only in position 1) means: Memory subsystem configuration activity is in progress. No memory modules were detected. Maybe the problem is only in the memory module.   Whad DIMM did you tried to use for the upgrade?   If this does not fix the problem, you need to try a Minimun to Post. Remove all components but the ones below and check if it boots:    System BoardProcessor with Heatsink1 DIMM in slot 1Riser CardPower SupplyControl Panel and Cables     Regards.
I have had a PE R210-II for some time. It's been running normally for a few years. I recently wanted to upgrade the memory. I powered down, installed an extra DIMM and afterwards the machine will not boot, no fan, no POST, no video. There is a single flashing Amber LED on the right side of the 4 LED display. I replaced the the memory to its original state (single 8GB DIMM in slot 1) and the state is still the same. I pulled out all peripherals (USB SSD disk, 3 SATA drives, DVD drive, ethernet cable) and the result is the same. Any ideas for next steps?    
R210 II no fan, no POST just a single amber flashing light