Great answer from Mark T. Sometimes when conducting this procedure, Lightroom loses track of the connection between your catalog file and your actual photos, in which case your photos folders in the left sidebar will display a question mark, or if that disconnect exists and you try to enter the develop module with a "missing" image, LR will alert you to the issue and won't let you edit until the original files can be located.
Fortunately this is an easy process too. Simply hover over folders with that question mark, right click and select "update folder location" or "find missing folder". You can then navigate to the new location and select the folder, at which point all is well again.