"App not working" doesn't give us much to go on. Is it the My Account app, or one of the others? What kind of problem are you having? What device are you using? Any error messages? The more details you can provide, the better.
Thanks for letting us know and apologies the app is not working as it should! Could you try uninstalling/re-installing the app following the steps below:
Go into settings > apps > Xfinity MyAccount app (may need to tap "Storage" to get to next step)
Clear app cache and data.
Once cleared, uninstall the app.
Reboot your device after you uninstall the app.
Once the device has booted back up reinstall the app.
Try to log in and let us know if it's still not working.