In the SIMATIC Manager environment you should refer to:
PLC Menu -> Diagnostic/Settings -> Hardware Diagnostics
Then in the opened window select your PLC and click on "Module information" button.
In the "Module Information" window you can move on to the "Diagnostic Buffer" Tab and from there you can see the status of your PLC and all the events (or faults) with their exact time and date of their occurrence.
From there, if click over a specific event you can see its details in a small area on the bottom of the "Module Information" window. So that you can figure out what is the reason for that particular event or fault.
Hope it helps. Let me know if you have further questions