Motion Sensor stopped working. Removed it from system and can't re-add it

If you have interactive monitoring, and if you uninstalled it from your Devices in your online control panel, did you click the "save changes" button afterward?  (you can see the sensor in your Device list if you forgot to click the "save changes" button).

If you indeed correctly uninstalled it, try reinstalling it using your keypad, as you did when you first set up your sensors (the keypad prompts you).  If this doesn't work, may need to call SS for troubleshooting.  You can also try taking out the battery in the sensor and just re-seat it and try adding it again (or as Shiherlis suggests, replace the battery completely).

By chance, did you check the batteries? Although just three months old, many customers have been complaining of receiving low batteries with their new systems over the past 2-3 months.