No boot sequence; One Low tone beep. Post-RAM upgrade.

One beep usually indicates that there is no RAM installed. First try booting the Mac with no ram in it to see that these beep codes are the same. If not, it would be a different issue.

Try holding CMD+Option+P+R to NVRAM reset with the new RAM in. Hold it until you hear the second chime.

Make sure when you handle RAM that you’re not touching the gold contacts because the oil in your fingers can get on it and sever the connection. If this happens, try swabbing the contacts with a Q-Tip and some rubbing alcohol.

It could also be that the RAM you got is bad and you might need a replacement. Since you said it boots with the old RAM, this rules out the DIMM slot being broken.