I renamed a folder and now I lost all the emails in the folder. Folder is grayed out. What happened? Can it be fixed

First thing I'd do is use the provider's webmail interface, if available, to see if the folder is still there as either the old or new name. While there, I'd make another, temporary, folder and copy all those messages into it for safe keeping. The copies can be deleted later. (This is the Windows version...) In Thunderbird, right-click on the account name in the left pane, and choose "Subscribe". You'll see a list of all folders. Click the "Refresh" button. Hopefully the missing folder will now be listed. Make sure there's check mark by it. Choose OK. If the missing folder now appears and it has all the expected messages, you can delete the temporary folder.