sent mail folder disappeared

macOS 11.0.1 running mail 14.0 on a mac mini. woke computer up from sleep, checked new email, could not toggle from one message to another by clicking on message line in inbox list. messages clicked on would not highlight or display contents; cursor icon in the form of a hand rather than an arrow. clicked on inbox in sidebar list and sent folder disappeared. cannot find any menu or preferences option to restore it. folder was not displaced into any other folder (inbox, trash, drafts, etc.) email server is on a POP account.