UpdraftPlus features a resumption feature – if you wait 5 minutes and visit a page on your site, then it should re-try not-yet-uploaded files. So, don’t assume that backing up has failed if a few minutes pass without any apparent progress. Be prepared to wait an hour and then check again.
You can also look for the log file (you can find it in wp-content/updraft, if you are using FTP). Before asking for support, you can improve your chances of a speedy answer if you can make sure that you: 1) Have started a backup, and then waited at least an hour (because UpdraftPlus will keep trying) 2) Have tried to find the log file of the failed backup.
Posted in: Backing up