I put fresh batteries in, put the base station in program mode, hit the Code button with a pin, hit Test to register it and it beeped once. I was able to arm it and it worked fine that night.
It can be easy enough to register a motion sensor accidentally... registering it with a new code... and the console will never be able to find the old one (even though it is really the same one). Clearing the consoles memory and reprogramming everything will be the only solution.
I've wired most of my DS10A sensors to a location near the console. So I can gather together my (MS10A) motion sensors.. as well as my remotes... and reprogram the console pretty quickly.
It's important to know that
the battery's last a nice long time in these units. It was more likely that the unit lost contact with the console for some other reason... like distance from the console.
I test and/or replace batteries in my home.. top-to-bottom from smoke alarms to remote controls.... twice yearly when we set the clocks back and then again in the spring when we set them up (forward).