Why does not see the RAM (even the old one) after adding the bar?

Question

RAM

There was one bar for 8GB (2133). I decided to add another one, also for 8 GB (2600 with the expectation that it would simply lower the frequency, 2133 was not in the store). After that, the computer started to start, but there was a black screen and the keyboard and mouse also did not work. Turned off and started to turn on again, after that it squeaked: 3 short and 1 long. Then I tried to start it just with the old bar, but nothing has changed, it still beeps. As a result, I checked both the new and the old RAM in turn in all 4 slots, but it still does not start. Tried resetting the bios by removing the battery, no change. I can’t find out what kind of motherboard it is because the computer does not start. I only know that the socket is LGA 1151 and that's it.
Could something be on fire? There was no smell or any external signs.