HELP!!! I'v lost my files!

Hello,
I had a little problem with my external hardrive and now i cant access it unless i format the hardrive, i understand that i have lost all my data at this point but i also understand that i can retrieve them all, i have installed 'Undelete Plus' but it cant read the external hardrive which leads me to believe that i must format the hardrive in order to retrieve the lost data, which brings me to this question...Will formating the hardrive cause me to lose all my data, or is that the next step?