Every so often I will get a user who writes in to the help desk saying that their "Preview" button is not working. Here are a few things to check:
Try previewing your prototype in other browsers.
Try Generating HTML files ("Publish > Generate HTML Files...")
Axure RP's Preview feature works by creating a web server on your local hard drive and serving up just the page you want to preview in your web browser of choice. If generating HTML files works fine, it may be that something--e.g. a proxy--is interfering with your browser's ability to access the localhost at "127.0.0.1".
There should be an option--either through the browser or through your OS settings--to bypass the proxy for local addresses.
If an IT department manages your workstation, you may want to ask them if they know of any thing that could be blocking this feature.
Here is more info on that:
visual studio 2010 - Localhost not working in chrome and firefox - Stack Overflow
If you are in doubt, this doesn't work, or the symptoms are different, feel free to write into support at axure dot com!