What would A 1974 one dollar bill be worth there is only one side printed the back side has nothing printed on it at all?

If it is a true error note and in uncirculated condition, it could very well be worth a few hundred dollars; less, if it's lightly circulated, but still in nice condition.

To have it authenticated as a true error (as opposed to a fake), you should show it to a minimum of 2-3 trusted coin shops in your area, and see what they'll offer you for it. (I recommend 2-3, because you may get substantially different offers from each!)