I work both on an iMac and on a Windows10 based PC and I use a 3 button mouse on both systems. I’ve returned to this section of the course because of an issue I am having with one of the .blend files I am working on. For some reason I am unable to zoom and pan using my middle mouse button on either system. I have no trouble zooming and panning in other files. I’m guessing I inadvertently changed a setting in the file and saved it that way but I have not been able to find where to fix it. I would very much appreciate any help I can get here. Thank you!
Hey Jeanne,
Press the Home button for the whole scene to reset the view for zooming/panning.
If a single object: Try pressing Forward Slash ( / ) on your Numpad and then press it again, or try pressing the Delete/Period button on your Numpad to focus on the object.
I think one of those should help you out 
Take care~
Pressing Home reset things and all is well. Thank you!