Classic problem. Your elasticsearch server crashed due to load or being out of memory.
The most likely reason is AI scrapers.
I restart the elasticsearch process nightly because it's so unstable.. opensearch uses less memory and is a lot more stable.. it should also be protocol compatible.
Since you have a server, i'd recommend installing netdata.
There is a ubuntu package for it.
Let me know if you could use help with the nginx or apache web service forwarding part.
At the time of the crash, you probably see one of the following:
elasticsearch dies
big memory fluctiation
too many simultaneous TCP/IP connections
It could be a number of problems so it's helpful to have netdata to diagnose them as you won't find much useful evidence in logs when elasticsearch goes belly up.
Something is not right or correct. There are so many ways to push / use ES. The latest supported ES is (I think) 8.x. ES is actually up to 9.
The latest ES is 9. Not really supported but works. My site a bit small but cranks.