Sometimes things "go south" and the program, or some part of it, stops working the way it is supposed to work. Trying to figure out the precise thing that is wrong can often take more time than it is worth, particularly if the issue is not one that has presented itself to you before. The usual suggestion is to run a repair on the installation, to see if that fixes the issue, and, if not, to do a reinstallation and, failing that, a clean uninstallation and reinstallation. (I usually jump straight from the repair installation to the clean uninstall/reinstall, as I have found few issues that a repair installation would not fix that did not require the clean uninstallation.)
One thing you can do before doing a clean uninstall/reinstall, is to see if the problem persists when a different user logs into the computer. If the problem does not occur for the new user, the issue may lie with user-specific files or settings. After backing up any and all customizations that user may have made, you can "reset" the user's profile (deleting it and letting AutoCAD® recreate it by re-running the secondary installer for that user) by following this procedure.