Time Machine isn’t backing up at all

Check the drive is connected and shows in Finder. If backing up to a local drive, use a Mac port rather than an external hub. Don’t use a USB port in a display that’s set to sleep; that will disconnect the drive. If Time Machine backs up to a NAS drive, ensure your Mac connects to it at login: in Users & Groups preferences, click your account, Login Items, then +, select the drive and click OK.