Hi, Go into the MENU, then scroll to "Setup MENU", scroll down to Reset to Default and press OK/ENTER.
Then you will go to the Channel MENU, and select "AIR" (instead of Cable).
Once done go to "Channel Scan" and hit OK/ENTER.
These are the steps that need to be done in order to get your TV to scan for, and pickup available antenna signals.
If that still does not work, then it would seem the TV's internal tuner has malfunctioned, and a workaround would be to purchase an inexpensive DTV converter box and connect it to the TV's AV Inputs. Then the scan will be run using the DTV converter box software.
The manual for your Coby is below:
Additionally, a DTV converter box can be purchased at Radio Shack, or Best Buy, and also at the link below:
If you need further assistance let me know. Please do not forget to click the 5 STAR RATING, at the top of the page for my efforts. It is my livelihood and you can follow up at any time if needed.