Have you checked the logs in the 12 hive to see if there are any
pertinent error messages when you do this? Also, you might check the
application logs (Event Viewer) of the web server and the database server
(apparently sometimes this can result from overlarge transaction logs).
One more suggestion: try browsing to a page in the SharePoint browser
GUI (Master pages seem to work well for this too), open the dropdown menu
for one of the pages, and select Edit In SharePoint Designer. Sometimes
this can shake things up enough to get you going again.