Modules are simply compilations of other Canvas content. To re-add an item (page, quiz, assignment, etc.) that you accidentally removed from a module, go to Modules, scroll down to the module you accidentally deleted an item from, then click the + button (beside the gear icon). Use the pop-up menu to select the item in question. For more on adding new or existing content to modules, visit https://community.canvaslms.com/docs/DOC-2827.