My Microsoft Office 2011 for Mac has stopped working…Help!

I have Office for Mac Home and Student installed on my macbook and until recently it has stopped working and acts like I need to purchase it again. I already used up the free trial it offered so I could get a paper done, but that has now expired. When I click on either the Word, PowerPoint or Excel icons, a window pops up with the options to either purchase a product key or enter a product key number. When I try to purchase it, I can only purchase the upgrade since the Home and Student is already installed. The upgrade to the business edition is 200+ so I'm trying to avoid that! Any suggestions?