Edge Loop (Alt + Click) not working in Blender 4.5 & 5.0.1

If you’re not on a laptop, can you try plugging your mouse and keyboard on a different computer and test thingsthere? Just to see if it’s not your peripherals that are broken all of a sudden? 

Martin says it may be hardware breaking, but it’s hard to diagnose from a distance.

Maybe even try on a couple older Blender versions, to really make sure is not a recent bug that is persisting? 

You’ve tried on a default cube? Maybe what you’re trying to alt-click select isn’t an edge loop, because the default cube doesn’t have edge loops.
For the alt-click to work you have to select edges that are connected end-to-end in a line. When you hit a pole, like on the cube where the edge ends on a vertex connected to three edges, then the loop selection stops there.
You can read more on it with examples here: https://docs.blender.org/manual/en/latest/modeling/meshes/selecting/loops.html

Sacha is right, but ALT+Select, can also Select Boundary Edge Loops, like here (Default Cube, top Face Deleted):

Terminology: a Boundary Edge, is an Edge that is connected to exactly 1 Face.

Maybe that’s where you get confused and say: it worked before.

Mystery solved. I’m glad your mouse and keyboard are just fine. 

“Silly beginner confusion” is a big reason we are here to help! Blender has so many little things that can, and will, catch you up sooner or later. Learning how to troubleshoot, what questions to ask, and how to ask them often seems like half the process. You did an excellent job breaking down what you tried and that helps so much!