All of my custom sheets are empty.

The entries still exist in the dropdown menu, but when I click on them there are no character or NPC sheets and no options to view such sheets. I am the GM of the game and there are no other players because I was using this game as a repository for the sheets I entered into Roll20 manually. These included transcriptions of monster stat blocks found elsewhere as well as original creations by me. I am logged in as the GM and am not sure what the problem is or why the sheets I spent a great deal of time entering are now either inaccessible or missing.

If you mean that the Character Sheet tab is missing, but the Bio & Info tab and Attributes & Abilities tabs are visible, then you need to check to make sure the character sheet isn't set to "None" in the game settings.  If it is something else, maybe some screenshots would be good to understand what you are seeing.

You have solved my problem, thank you! I don't know how that setting would have been changed, but changing it back restored my ability to see my character sheets. I appreciate it.