Incidentally signal strength should not be an issue thanks to the booster in the loft - with a multimeter I read 9V off the aerial.
If the power supply is fine, and this happened with a previous card as well, then I would suspect the booster, or maybe connections have gone bad. Check and clean all connections in the signal path from Antenna to the tuner.
Also, if your antenna isn't getting a signal, then boosting it will just boost noise.
It looks like you are in America so I'm no expert being in Australia, but over here when we went from analogue to digital transmission nearly all of us had to change our antennas to get a good signal. Are you using an antenna designed for the signal you a receiving?
Does the manufacturer provide any software to watch TV, and if so, does it work?