I have a Maxtor external drive that worked fine when I stored information on it, but a few days later tried to place the information back onto the hard drive of my computer and the Maxtor is not recognized -- makes a clicking sound.
your HD died. the clicking noise is the arms trying to cross the platters and failing. your data is lost unless your are willing to spend the money on data recovery. its a safe bet that a basic starting cost for such a recover would be 700-1200 USD. every now and then i get a drive that i can hold at just the right angle to make it load and quickly copy data off, perhaps you could get lucky.
as for why this happened... did you drop it? did you move it while it was on? did it fall over while it was on? is it very old?
if its new you can RMA it for a new drive but they will not get your data back.