itunes undoing an action

Unfortunately there is no undo for your conversion and you don't say what you converted from or to so the solution may or may not work for you. Presuming you now have two of everything but in two different formats (for example AAC and MP3) you could sort on the Kind column and then delete the ones you don't want. In iTunes library view, click at the top of any column, from the list choose Kind. Now click at the top of your Kind column and sort by format. Go to the first track affected by your conversion, for example MPEG audio and click on it, hold down the shift key and got to the last in the list and click on it. This will highlight the selection which you can then delete.