all my purchased songs from Itunes says unable to locate original file

Did you recently move any files around on your computer. iTunes looks for the files in the original location; if they are moved this can create problems. One possibility is to right click on one of the songs with an exclamation point, and the go to "Get info." This will open a pop-up window that has several tabs.Click on "Summary." At the bottom of this screen, there is a section that says "Where:" This is the path of the original file. Check that location and see if the file is located there (probably not, or you wouldn't get the warning).

Next, I would search Windows for the particular song. Assuming that you find it somewhere, I would bet that is where the rest of the missing files now reside.