Commercial service and installation technician, been with the company 8 years doing technical troubleshooting and installations for residential and commercial accounts. Im a front line tech and get to see more information of problems than people sitting behind a desk using scripts to help you... Follow me i can help.
Try to do a manual reset on your box unplug the per chord and wait like 15 min. Then try again. Afterward if that does not work. I suggest calling comcast you may have emergence on your cable system which can cause these kinda of problems. This also causes tiling and missing chan. If this is happening call for a tecch to check out your lines. A chamge of service like down grading can also cause this.
How many cable boxes do you have that are On Demand capable?
Are you only having issues with one of them?
Sometimes being behind on your bill will cause your On Demand to stop working. Also as stated above, if you have issues with signal or bad cable you can also have problems.
If you are having issues with all your boxes, you may need a tech out to verify the signal. If only one is having the issue, you may want to check that the connections are tight. Resetting your box may only temporarily resolve the issue. If you do reset it and the issue continues, you will need a tech out to fix the problem.
If you connected a gold Radio Shack splitter to hook up another TV, this will cause On Demand and internet to stop working. If you did install one of these, please remove it, reset and verify if it works.
Billing to tech support, I'm your chick! I've been with the company for over 7 years and I am a REAL, LIVE Comcast Employee. I'm not a vendor and I'm not an old employee. I'm now...in the moment. Let me help!
All of these answers are giving way too much information that may or may not relate to your problem. You mentioned you just started On Demand for the first time which tells me that you are somewhat of a new customer. Not to worry, this issue can happen from time to time.
Master Kable's suggestion of unplugging the power cord for a short time and then trying again after about 15 minutes is the best thing to try but more than likely, you will need to call in to have a refresh signal sent to your cable box so that it can properly update with your account information, especially if the error code is S0A00.
If you have any further questions, please feel free to ask em directly http://www.insidr.com/in/misgee and if you believe my answer was helpful, please mark it as such.