No reception from antenna or cable:
Make sure your source is providing a signal and that the cable connectors are good (center pin not broken or bent). Try another TV if possible.
Make sure you source select switch or mode is set correctly. Someone may have accidentally set it to direct video or AUX input.
Are all bands affected? If so, the tuner or IF is faulty. If there is a lot of snow, then it is probably toward the front (circuitry wise) of the tuner. If it is just a black screen, then it could be in the IF or video amplifier.
If only certain bands are bad - channels 2-6 for example, then certain parts of the tuner circuitry are faulty. However, make sure the CATV mode is set correctly as this affects reception on a band-by-band basis.
The problems may be due to bad solder connections of the tuner shields, connectors, coils, and other components. Try prodding the tuner to see if you can make the problem come and go or at least change.