Adobe Premiere Error: Unable to Import MPG or MPEG File
Occasionally you may find that Premiere Pro fails to import or open MPG files. For new files, they show as "unable to open" or "unable to import". This can also happen to MPG files that you have previously imported and used without any problem.
Try deleting the associated xml metadata file (it will be a file that has the same name, in the same location, but ends with the extension .xml). Re-import the file and see if it works.
In our experience this works about 50% of the time. If it doesn't work, you can either be patient and try again the next day (seriously, we've found that this often works), or reinstall Premiere Pro using the instructions on this page.