Videos not export. No errors

Hi, I have been using Premiere Pro for a few years to stitch videos together. But in the last few months lots (but not all) videos won't export. If a time somes up in media encoder it just sits on 22768 something hours to go. There are no errors (so no errors or crash reports to share). I have already worked my way through the suggestions given on the help pages (e.g. updates, changing settings, different file formats, etc on How to fix issues that cause errors when rendering or exporting). Anyone ever had this problem and found something that worked? Thanks