It might help if you explained your situation in more detail.
There may be many reasons why you are 'unable to open...' any of your documents. It would help if you explained why? What happens when you try? Do you get an error message of any kind?
You also say you have an iMac from 2009 - does that mean you're using Microsoft Office/Word 2008?
If so, that could be your problem right there - you need the latest version of Office - Office 2011 - to work with Mountain Lion. You don't say what Mac OS X version you upgraded from to know if that's an issue or not.
Yes, you will.