In admin > process queue, edit the status of the video and make it convert again.

Then run the cronjob by visiting the url

http://yourdomain/cron.php

After doing this, video will get converted (if more videos are on queue, you need to call cron.php few times). Then chk the debug log (templates_c/debug.txt) and also the status of the video in process queue.

If video is not getting converted, see

Video not getting converted to flv