The filter lists you are subscribed to are updated automatically on a schedule set by the list authors. AdBlock will try to fetch the updated files as long as your browser is open and AdBlock is enabled and running. The AdBlock extension will also try to fetch updates when you open your browser.
If a web page suddenly isn't working or doesn't look right, it might be caused by a broken filter in a recent list update. If you're seeing ads, your filter lists might be outdated. In either case, making sure you have the most recent filter lists is a good first step in troubleshooting.