Hi Leslie, since you didn't disclose the brand or model name and number to the antenna you are using, I'll do my best to help you with your dilemma.
1.) double check your physical (wire) input connections
2.) make sure you have selected the proper input from you t.v.'s menu screen
HDTV aerial (outdoor mast) antennas are your best investment to receive free air wave digital transmissions...as opposed to the smaller indoor antennas...and the clarity is phenomenal; it cannot be matched by cable or satellite signals that compresses and decompresses the audio and video signal.
Why? Because, digital t.v signals are line-of-sight signals; meaning they need an unobstructed path between the sending source (t.v. station transmitter) tower and the receiving (home) antenna.
Hills, trees, walls, buildings, and any other obstruction- bends, deflects and weakens the signal as it is...placing an HDTV antenna in an attic weakens the signal by at least a 1/2...
So, if you are using an Indoor omnidirectional antenna at the set; the signal will be even weaker...it won't be good enough to pick up the digital signal that requires an accurate line-of-sight reception (just like satellite dishes.)
As a side note: I had an outdoor HDTV mast installed professionally; including signal filter and signal booster- [they'll know the proper type antenna to use for your area (see http://transition.fcc.gov/mb/engineering/dtvmaps/
for more details.)] And it was so worth it; whenever we have storms that knocks out cable power, we rely on the air-wave stations...and their sub-stations, lots of choices; and it's phenomenally clear...much clearer than cable or satellite signals can ever be.
As a matter-of-fact, we watch our local news and entertainment stations normally via our antenna rather than our cable service provider's system for that very reason of clarity and detail. Too bad we couldn't get the quality "pay for" stations that way, too - we'd drop our cable service and save a lot of money! :)