Logging and Monitoring are important factor for system admin. Checking the log will help you have a closer look into the issue. One tool could help you will handy features is journalctl.
Here are simple options :
-f: follow the log (tailf)-u [service]: filter to show only [service] logs--since=[date]: Show entries not older than the specified date--until=[date]: Show entries not newer than the specified date
Example :
$ sudo journalctl -u nginx.service
$ sudo journalctl -u nginx.service --since yesterday
$ sudo journalctl -u nginx.service --since "2018-01-01" --until today