I've tried searching for this particular problem, but everything I've come up with doesn't exactly match this particular issue.
We have uptimerobot checking our sites for us, but on two sites that we use will report that the site is not working every now and then. It's completely random, and other than Apache not responding to requests, the system itself is running fine.
Both systems in this case are Server 2012 R2 VMs, and CPU/RAM utilization is a non issue. (<5% CPU, RAM is around 1.3 GB in use out of 4GB allocated)
If I bounce the Apache service either via Services snapin or the XAMPP control panel, everything works fine again.
The logs don't show much, but maybe if I had a better understanding of what I needed to look for... I could be more useful in that regard.