The Montgomery County Department of Public Safety has changed the frequencies on which emergency responders' radios operate as part of the federally mandated rebanding project. The scanners that some people use to monitor those frequencies will have to be reprogrammed to the new channels to continue working.
Montgomery County was required to make these changes by the Federal Communications Commission to eliminate interference that may be caused by the cellular phone industry.
A document with the frequencies and the trunk tracker ID codes can be found here: Scanner Frequencies
The link below contains additional scanner and rebanding information you may find helpful.