There are a few reasons why your On Demand service might not be working. First, check to see if there is an outage in your area by visiting the Outage Center on Comcast's website. If there is an outage, Comcast will provide updates on when service will be restored.
If there is not an outage in your area, the next step is to reboot your equipment. Unplug the power cord from the back of your TV box and wait 30 seconds before plugging it back in. Once your TV box has restarted, try accessing On Demand again.
If you're still having trouble, you can try resetting your TV box by pressing and holding the Reset button on the front of the box for 15 seconds. This will restore your TV box to its factory default settings and should fix any issues you're having with On Demand.
If you've tried all of these troubleshooting steps and are still having problems, please contact Comcast customer support for further assistance.