thanks for your help, I also thought the same but the server reboots only after the SQL Services are started so I thought SQL side to be checked before we...

That's right, that's all I see in the error log. It is strange that nothing is recorded in the error log file.

The server went into a continuous loop of...

2015-10-26 18:40:19.77 Server Microsoft SQL Server 2012 (SP1) - 11.0.3128.0 (X64)

Dec 28 2012 20:23:12

Copyright (c) Microsoft Corporation

Standard Edition (64-bit) on Windows NT 6.2...

Yes, I did check that and didn't see anything relevant.

I see the following in SQL log file

Error: 18456, Severity: 14, State: 38.

I see four critical errors in Event...