Things don't just HAPPEN randomly. Something changed. Traffic changed. Code changed. A value in the database changed. Solving the problem is the art of solving what changed. If you can articulate the difference between what worked before, and what does not work now, the solution almost always presents itself.

Scenarios include;

What template file is that page or block using?
How to use xDebug and XhProf
Lovely logs - what’s in them and good ways to collect them
Devel Module & Memory Profiler
Advanced developer tools (such as console.log, Network panel)