Is your WordPress homepage loading fine and your other pages are showing a 404 error? No need to panic, this error is usually always solved by going to your WordPress Dashboard: Settings > Permalinks and click Save Changes to give your permalinks a refresh.
This is not your every day situation with a WordPress blog, but sometimes after activating a plugin or a plugin update that may have affected your permalinks and they need to be refreshed.
If you are getting this error unexpectedly a good place to start troubleshooting is by refreshing your permalinks.