Marco, can you recall doing anything up to 24/48 hours before the problem occurred? You may not have noticed it immediately because the old page may have been in the cache or the browser cache. Did you make any changes to any files regardless of how small?
If it still plagues you, it's possible you may need to upload the core minus /includes.php, the ow_plugins directory and the ow_templates directory.
You can do this rather painlessly by temporally closing down the site if you wish. Just pick a quiet time and rename your site folder. You can add a hyphen in front of it like this -mysite. Create a new site folder and upload the Oxwall files into it. DO NOT ATTEMPT TO INSTALL THE SCRIPT. Instead, copy /ow_includes/config.php and the ow_plugins folder over to it. That will add the original config details and all your plugins. Do the same with the ow_templates folder if you're using a 3rd party template or modified the default one.
Try running the new site. It should work. If the problem persists, it could be the result of a plugin or template playing havoc.
If the site doesn't open, you can always rename the new directory and remove the hyphen form the original one. That will get you back to where you started.
Always backup everything first!