OK, first let's start with the BI log itself. The default location for the log is 'C:\Program Files (x86)\
Blue Iris 3\Blue Iris.txt' although it may be different if you've changed the defaults. It may be big - too big for notepad - depends on how you're archiving it and the leangth of time you've been running BI. Look for lines similar to those below:
0 12/13/2014 5:27:35 AM App Started
0 12/13/2014 5:27:35 AM Clips New: storing 2 files 3.14G [2/2, 476477/512000, 476477]
0 12/13/2014 5:27:35 AM Clips Stored: deleting 1 files 1.34G [3/5, 476477/307200, 476477]
4 12/13/2014 5:27:35 AM Clips D:: 208, 596.6G/800.0G (+263.3G)
4 12/13/2014 5:27:35 AM Clips C:: 0, 0K/9.76G (+33.5G)
0 12/13/2014 5:27:37 AM Server Started on port 80
0 12/13/2014 5:27:37 AM App Today's sunrise: 7:48 AM, sunset: 5:08 PM
These are the lines indicating it's startup. Also look at the lines preceding it, they should indicate the shutdown reason, a normal shutdown would be like:
0 12/13/2014 3:15:38 AM App Stopped
If it say's something like "Unexpected shutdown" then it was a "crash". It may give more hints - but many times it does not. What we are interested in is the time it happened (in the example that would be 3:15:38 AM), this is the time we will be looking around in the event viewer.
Now onto the Windows event viewer. Go ahead and open up the event viewer and select "Create Custom View" from the right side menu. Once that opens on the "Filter" tab change the folllowing:
Logged:
Select "Custom range..." and in the pop-up choose "Events On" and set the "From:" time to be 5 minutes before the stop time noted above (that would be 3:10:38 AM using the example time given above). Do likewaise with the "To:" time, except make it 5 minutes after (that would be 3:20:38 AM using our example time). Select OK to save and return back to the "Filter" tab.
Event Level:
Check ALL boxes EXCEPT "Verbose".
By Log:
Choose the "By Log" option and and in the "Event logs:" dropdown select "Application, Security, and System from under the "Windows Logs" tree (it has a little plus you can click on to open it).
That is it, click on "OK" to save and give it a name (whatever you like). You should now see the custom view you have created on the left tree menu under "Custom Views", click on it to check it out.
You can widen the time window and the number of logs it searches - but the ones given should do the trick. Hopefully you will see something that stands out in that time frame to give us a further clue as to what is happening.