FYI for the future this should go in the AppMon and UEM forum - Dynatrace is a different product.
The logs will be on the collector the monitor is scheduled to run on - in this case you might not see any because it is not running. Is it actually suspended (with the pause button icon) or does it have a question mark? Right clicking -> details on the monitor might give you some information if there is a reason it is not running.
After we restarted dynatrace servers's services at weekend, status changed it is not suspended any more. But this time it is in "incomplete" status and there is no question mark and no detail just below info:
Last Run Result:incomplete
duration last run : 29s
Detailed message:Rowcount: 0
Retrieved measurements:Connect Response Time: 0.0
DB Connection: 0.0
Query Response Time: 3.668310003949288E9
Response Time: 25631.097272
Row Count: 0.0
You can access logs via Dynatrace Client, System information -> Connected Systems then you can drill down to the Collector on which the DB query is scheduled. Then select log files, file name would be com.db.tabmon.monitor.log