Have you selected something ( a clip, a sequence etc.) to export before going to the menu File > Export?
Sorry for the trouble. If the issue persists after installation and following step recommended by MtD, reset the preferences using the steps suggested in this article: FAQ: How to reset (trash) preferences in Premiere Pro?
Meg's suggestion worked for me. I didn't have anything selected. Click and drag over your assembled video / audio clips to select them. They will all turn white when selected. Then chose Export/Media and follow the prompts and choose the options from there to start your export. Hope this helps!