First thing to try is simply cycle power. If you've temporarily avoided a channel, it is restored after the power cycle.
Second thing, if you programmed your scanner from a computer, just re-connect the two. Then do a "Write to Scanner" and the program "stuff" in the computer will overwrite those button presses. (Do NOT read from the scanner first - doing that overwrites the "good stuff" in your computer with the "bad stuff" in your scanner.
To program a frequency by hand (computer is better though):
Menu>Manage Favorites (scroll to the FL where you want the channel)>Review/Edit System (scroll to the System where you want the channel)>Edit Department (scroll to the Department where you want the channel)>Edit Channel>New Channel {for a new channel} or scroll to the existing channel. From there, you enter/change the appropriate information.