Multi Monitor Setup

I have three monitors connected to my rig, 2 x 20" in portrait on the sides and a central 27" in landscape. For months I have been using this without issue, today however when I woke up the 'main display' (i.e. where the start button and taskbar is) has shifted to to the left display. Nothing I do will seem to fix this, Windows thinks the central monitor is still the main one according to the display settings. Restarting (the traditional fix for anything!) doesn't help.

Not sure what else to try!

Could be a Windows Update caused the change, you might consider doing a System Restore back 2-3 days and see if this resolves it, if it does stop automatic installation of updates and set it to be asked then do them one at a time so you can find which one did it.

have you tried simply dragging and dropping the monitors (in the screen resolution dialogue) to the correct position?