Kevin,
Can you be more specific. Do you mean the scanner won't power up, or do you mean it says "Scan mode: nothing to scan." If it says the "Nothing to scan" go to menu, scroll down to "weather," then press "weather scan" and see if you have anything. If you do, then you know the unit is working (antenna is connected, etc).
The most common reasons for geting a "scan mode: nothing to scan" screen is 1) the database or favorite lists in the Set Scan Selection are turned "off," 2) the "Select Service Types" are not on, or the ones are not "on" for the channels you have programmed, and/or 3) the "quick keys" are off.
Is your SD card OK and able to be written to Sentinel software?
Please be more specific about what you did between the time it worked and did not work, and I'm sure some of us will be able to steer you in the right direction.
Steve AA6IO