I'm sorry to hear about your mother's fall and hope that she wasn't seriously injured. Yes, there are magnetic monitors or sensor pads that are designed to alert caregivers when a senior leaves the bed. They are sometimes called bed alarms, and you can find them online or in stores like Wal-Mart.
You will want to do your research before purchasing, as some bed alarms use different kinds of technology like pressure sensors or infrared sensors. Some will be wired, while other will be wireless. Some are powered by an adapter and others are powered by batteries.