Cable boxes are finicky. Most problems with them can be fixed with a hard reset. Here are the instructions for rebooting a Scientific Atlanta DVR cable box. These directions have been tested on cable box versions 8240, 8300, and 4200.
Two methods exist for rebooting your Scientific Atlanta Cable box.
The first is to press Vol+, Vol-, and Info buttons simultaneously on the front of the converter. Once the box reboots, you will probably have to press the power button to turn it back on.
The second method is old school. Turn off the cable box with your power button if possible. Disconnect the power cable from the back of the device. Wait 30-60 seconds and then reconnect the power cable. Once the box finishes restarting, you will have to press the power button to use the device.
Occasionally, the second method is the only thing that works if the device is completely locked up. The reboot process takes quite a while. You will know it is completed when the time appears on the front of the device.
I believe this works on most SA boxes including the popular 8240, 8300, and 4200 versions. Please post in the comments if you have success using this method with other devices.