I have been using Lightroom CC with no issues until I imported a new jpg into my library last night. The thumbnail of the jpg looks similar to the color scape when viewed on my camera, however, when I click on the image to edit, the large version of the photo has muted and incorrect colors (almost like the white balance has been changed, but all sliders are set to 0). I have tried deleting the image and re-importing it changing the import preferences between "Adobe Default" and "Camera Settings" but neither option fixes the color difference (the issue remains the same). I tried editing the image to look like the thumbnail, but upon export the edited image is garish and totally off-color. This change is also present in my old edited photos (the thumbnail looks like the photo I previously edited with correct color scape, but when I open the image for further editing it is muted with a purple tint). I have restarted my computer, I have restarted Lightroom CC, and I have made sure that I am running the most up to date version. I even tried importing and opening the images in the Lightroom Classic Develop Module with the same result. To re-iterate: all my files have been imported as JPG, not RAW images, so the rendering of RAW images is not my issue. I am running Windows 10 on a laptop with a BENQ monitor connected via HDMI as a second screen for editing, which until now has been a perfect set-up since I got the monitor at the beginning of the year. The BENQ monitor is set to sRGB, but even if I change the color scape to Adobe or ProPhoto the color difference is still obvious between the thumbnail and the open file. Even though the quality of my laptop screen is terrible, the color differences are still obvious so it is not just a second-screen issue. Windows had updated just before I imported this most recent JPG, which is probably coincidental, but I am mentioning just in case. I'm at the end of my limited LR knowledge - what else can be causing this and how do I fix it?
The windows update probably messed up the color profile associated with your monitor. This is a common occurance on Windows unfortunately. I am assuming you are not calibrating your monitor actively from your description. What you should do is go into the monitor's properties panel and look for the tab that lists the color profile associated with the monitor and delete it. This will make windows assume that the monitor is sRGB exactly and should fix your problem if you also select sRGB in the monitor itself.
Best is to set the monitor to native by the way and use a hardware calibrator on it. BenQ provides software (palette master) that works with most calibration pucks to give you a profile that updates the LUT in the monitor itself so you get posterization free and wide gamut colors on the display with 10bit resolution (depending on the monitor). BenQ does a reasonable job calibrating the sRGB and aRGB modes on these out of the factory but it is far from perfect and if you want trustworthy color you have to calibrate.