I would make sure you haven't logged in to Garmin Connect with a different account. Otherwise, courses should not disappear unless they were inadvertently deleted.
If this is not the case, then if the courses still exist on your Garmin device, you can import them back into Garmin Connect one at a time.
Browse to the course files on your Garmin Device. In my case since I have an epix watch, my course files are located at \EPIX\Internal Storage\GARMIN\Courses.
Select the course you want to import
Follow the prompts to finish importing the course.
Please note that only courses visible on the currently loaded map will be listed, so make sure the map shows the location of interest, and is sufficiently zoomed out.
