I have a couple of ifs. If you're running Win 10, open settings and then open apps. If you have Microsoft 365 click on it, if not, look for Outlook and click on it. One of those two will have the option to repair or uninstall. Choose the repair option. Follow the instructions, one is a quick repair, the other fully reinstalls the app. Both options save all of your information, nothing is lost. These will fix any corrupt files or problems with minimal input from you. If these don't work, look at your server settings while Outlook is open by clicking file, account settings, then server settings. This will only work if you know what your server settings are supposed to be, and you are comfortable with doing it. If you are nervous, don't mess with them. You can find out how to fix many problems by going to YouTube and entering your problem in the search box. Someone will have made a video of that problem and how to fix it. Trust YouTube, almost any problem, no matter how ridiculous. The first part of my answer should take care of the problem.