I really don't know how to explain it properly but please bare with me,

What happened is that i've noticed that in "Latest data" all latest item checks results haven't been updated for several
days - while all items haven't been updated for at least 3 days but the first ones stopped updating 4-5 days ago randomly so that 3 days ago none of the items were updated anymore.

Since i had the zabbix_server running with debug logging the logs were overwritten and i could not go back and see if there was some initial error.
But from what 've saw in the current debug log there were no signs of errors, also zabbix server was communicating with the agents without problems, but one of the checks were executed also zabbix was communicating with mysql without problems.
After restarting zabbix server - everything returned to work ok.

Any ideas what could have gone wrong?

The bottom line : zabbix was not really monitoring for several days, but I had no idea about that since everything seemed to work ok.
The question is how can i automatically know that zabbix server got into similar bad state?

Thanks
Alex