CentOS7.6 server is rebooted suddenly without any action.

Usually the route is to check your logs for any "strange" messages..
If no useful info is pulled from the logs i guess its time to stress the server to check if some thing -hardware side- is wrong
Linux has a package named stress so you install it on your server and you begin testing...if nothing pops up then i would use memtest+ to check memory on more low-level and if nothing comes up then you should check your PSU (probably change it if you have a spare) and then inspect the Motherboard for any defective elements(capacitors)

First check your logs for a reason.

If someone rebooted from the console, then all you'll see is some phantom/random reboot out of thin air.

If this occurred, then you'll open a ticket with your hosting company asking for an explanation.

You haven't told us whether you maintain this server on your premises or rent it.  What you should do next depends on where the server is located.

Dr. Klahn brings up a good point.

If your server is on site, then you're they only person who knows what goes on at the console.

If you lease a server, then your hosting or provisioning company are the only people who know what goes on at the console.

Be sure to mention if you're running your server in house or you lease your server.

General info since as previosuly pointed out non are provided with the question
1) power fluctuation (a brief power interruption will result in the system's reboot)
2) which logs were checked, /var/log/messages?
if it does it repeatedly, your issue is power supply or memory. A faulty power supply that has fluctuating  voltages when the voltage across memory modules drops below the 10% tolerance, the system will panic and will reboot.

All system contrallable events will be in the logs.
Power related issues depending on the hardware you have might be in the hardware logs (i.e. powersupply lost) if you only have a single power supply, no event will be recorded.

Hi all ,

Many thanks for response .
There is no any power fluctuation or any infra issue , while other server's on same machine are working fine .
in logs no abnormality observed , please suggest further if any.

Thanks

a) This is probably a virtual machine ("other server's on same machine are working fine").
b) The other VMs running in the same machine are normal, ditto.

Therefore,

c) It is not a hardware problem.  QED.

Half the issue is solved.  This is a software problem.  Some possibilities are:

a) The VM might be crashing out of system state.  Unlikely if the other VMs are identical O/S and software and do not fail.
b) Somebody hates this particular VM and is shutting it down manually.  Unlikely if the times are random, the logs do not show logins and no shutdowns are logged.
c) There is a virus or malware.
d)  Some user software is using up all of a critical system resource, causing the VM to crash without warning.  Possible on Windows, unlikely on linux with the kill-processes feature.

Of the four I think that (c) is most likely.

Be great if you can answer Dr. Klahn's question...

You haven't told us whether you maintain this server on your premises or rent it.  What you should do next depends on where the server is located.