I'm not familiar with that one either, first try to reset it as others have said; but from what I found out about it, it's wireless, so you may have to pair it again to the sending unit. If that doesn't work then the sending unit may have moved and is either too far from the magnet, or not inline with the magnet, or that magnet has moved and it's not pointing at the sending unit. If that doesn't work then the battery in the sending unit may be bad.
Wireless computers are not known for long life, and the first thing to fail about 99 percent of the time is the sending unit. I've tried several brands of wireless jobs and all of mine the sending unit failed; so far the VDO has worked the longest, all my others would last an average of 3 years, the VDO is now 7 years old.