Hi. without more details it would be hard to say what is wrong. Are both installations the "Student" version? Do you have a valid student version license? If so, it would be less problematic to uninstall 2020 first, then install 2021, but usually they coexist just fine on the same computer. However, since you already had an error and now have two versions installed, it is possible that a corrupt file is being referenced by both versions.
I would go to your Windows "Apps and Features", select "Programs and Features" in the upper right and scroll down to the 2020 installation and right click on it and select the "change" option and select "Repair or reinstall", then "Repair".
Then I would repeat the same steps with the 2021 version and try launching 2020 again to see if it works.
If those ideas fail you should check to see if you have an AMD video graphics processer . If so it is likely a bad driver. See the following to find the way to fix that.
https://forums.autodesk.com/t5/autocad-forum/fatal-error-unhandled-access-violation-reading-0x0028-e...
and if that isn't it, maybe this:
https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/FATAL-...
ADN CAD Developer/Operator
