Premiere Pro Won't Let Me Export URGENT

HI All, 

I exported a 90 second video yesterday for a client no worries at high bitrate and 0.264 etc. The client had some small changes of which I made and now when I export the export page comes up and I put in my preferences and then the page disappears and it won't export. I have tried cutting and pasting into a new project, I have also uninstalled the program and re done it and I have also done a save as option.

Please very desperate for help. Many Thanks in Advance

Rachael