Server incident today

I can't explain what happened.

Apache http server
Got a call saying that the clients were not getting a response after
everything was going normally up until the middle of the afternoon.
All the http requests were showing up in the access log as normal. The
requests should have been written to physical files on the i, and nothing
was showing up. Option 13 on the CGI job was showing a list of user test
libraries. I am only used to seeing such a list during the execution of the
http request (while debugging and stopping the program), and then the
library list reverts to its default list. I checked to see if the data was
being written to files in a test library but did not find any. Still, this
did not explain how any user library list was still in place while the CGI
job was apparently waiting. I restarted the server and everything went back
to normal. This application has been running for around 15 years already.
Any thoughts on what may have happened or how we could better monitor what
is going on?