There's an article that can help you troubleshoot the issue you're experiencing with Pages.
Below is the section for a software issue that occurs in one application only:
For example, one application does not open or unexpectedly quits. Mac OS X v10.4 and later can help with applications that unexpectedly quit by offering to open the application with default settings. If this doesn't help, skip to "Drag the application to the Trash and reinstall it if the issue persists..." below.
- Make sure you are using the latest version of the software. Check with the manufacturer if the product is published by a third party.
- If the issue only occurs while using a specific document, try creating a new document or replacing your document with a backup. Your document may be damaged. If possible, try testing with either a new file or a known-good file.
- Drag the application to the Trash and reinstall it if the issue persists. Some Apple applications that may have been included with your computer can be reinstalled by using the Install, Restore, or iLife disc that came with the computer. See these links for more information:
Hope that helps,
Your profile says you are using Snow Leopard, but what you describe matches a common problem in Lion and Mountain Lion when the system tries to Resume a document that is no longer available. In that case, holding the shift key down while launching Pages is the solution. (And, in the future, closing your documents before quitting the application.)