I see an app in the Gallery is down (though not all) and another post mentioning an issue with the Streamlit Cloud service. Try rebooting your app. If this app worked previously on Streamlit Cloud without any changes and if the reboot doesn’t work, it is likely either an issue with Streamlit Cloud, or some issue with an upgrade.
In the later case, you could try to run your app locally with Streamlit 1.18.0 which was just released to confirm any compatibility issues if you don’t have Streamlit pinned to an earlier version in your requirements. (You can check if any of your dependencies had a major upgrade, too.) Otherwise, I’d try rebooting again tomorrow to avoid running in circles around an issue that may be outside of your control.
If this is a new deployment, please share more information about your environment and code. (Link to GitHub, for example.)