Digital strength low...what does that mean?

I’ve had a very basic cable package for several years now. We pay about $20 a month, get about 10 channels. We bought a flat screen TV, and suddenly, with no change to the cable package we had channels in the upper end. Most of them are the same channels as at the lower end, but with a better picture.

Well, came home from work, turned on the TV, and I’m getting no signal, just a message floating around that says “Digital strength low.” I went down to the lower end, that says it’s analog…but that’s just fuzz.

I’m current on my cable bill.

What is up with that?