You can also go through a little check list to eliminate the most obvious reasons for fewer birds at the feeders.
1. Make sure your seed is fresh. One way to do this is to pinch the seed on a piece of paper and see if any oil comes out. On cold days where every meal counts, if your seed has dried out your feeder will be skipped. (Wild Birds Unlimited receives a fresh load of seed each week).
2. Clean your feeders at least once a month. Make sure there is no mold in the bottom of your feeder. This can be dangerous to the birds. To prevent mold in bad weather use Feeder Fresh™ (a silica grit that absorbs water and humidity, is safe for birds, and made from non-toxic absorbent sand).
4. Look for predators. Hawks or cats can deter birds from feeding in your area for a short time.
5. Check with local birding groups to see if you are the only one reporting fewer birds.
I can only imaging the eeriness of having no birds. I'm always lucky enough to see a constant mix of changing birds. When I walk out to fill the feeders in the morning I hear the Whoosh of birds flying from the trees. Then as I fill the feeders they slowly filter down like falling leaves and hassle me to move quicker. I love it.