Illustrator gives file corrupt message while opening any existing files

OK. Here are a couple of things that are generally offered to fix odd responses with Adobe Illustrator. They range from simple to really invasive, so you want to try these in order:

• Turn off GPU Acceleration — If you're lucky, it may just be a glitch in your program's video response. Go to your Illustrator pull-down menu and select the Preferences>Performance... menu command. If GPU Performance check box is ticked, de-select it and try again. If the GPU Performance box is not, ahem, ticked off, that's not the problem.
• Clear caches and preferences — I usually share a link where you can find this online, but it seems that Adobe Illustrator now builds help file access through the program itself and there aren't any direct help links to be found in Google. So go to the Help>Illustrator Help... menu command and look up Clear preferences and Clear caches in the Discover dialog box. it'll walk you through a step-by-step process to do both these functions, which can often help with such anomalies. Of course, you'll have to write it down and do it manually, since it will involve closing the application before you can perform these functions.
• If that doesn't work, The Rude Cure comes into play — Get Adobe's Creative Cloud Cleaner tool to completely remove the Illustrator install, then re-install the latest version of Adobe Illustrator. If it still doesn't work, I'd suggest using the CC Cleaner tool again, then when you return to the Creative Cloud app to re-install Illustrator, click on the three dots to the right of the application line and install the previous version of the software.

Often, when Illustrator has problems it desn't so much crash, or stop working, per se. It's more like those silent movies where parts start falling off the car until you find yourself sitting in the middle of the street with nothing but your seat and the steering wheel in your hands. Hopefully going through the first two bullet points will get you up and running smoothly again. If not, The Rude Cure in the third bullet point may be in order.

Good Luck,

Randy