did it work immediately after you fitted the extra drive?
(lookie here to see how it "should" be done!)
http://www.pcanswers.co.uk/tutorials/default.a sp?pagetypeid=2&articleid=45459&subsectionid=6 06
(just grasping at straws - computer shops do sometimes overlook the obvious!)
if it did ... fo3 is almost certainly right ... "messing" about will often push a failing component over the edge.
If the disc has precious files on it ....
there are various recovery firms
or..... if you're feling lucky .....
there are a couple of things you could try .... but all are potentially destructive.
1 hit the disc repeatedly with a screwdriver handle ...
(could just loosen something? - worked on my old telly)
2 Put the disc in the fridge for a couple of hours or so
(the idea is that if the spindle bearings have seized the drop in temperature may contract it enough to get it working for 30 mins or so)
3 Open the disc up ....
(carefully ... don't touch the platter (the big shiny disc .... mmmmmm shiny)
waggle the arm (there will be a catch holding it off the platter - dont force it)
spin the platter
re-connect ... fingers crossed ...
none of these methods is guaranteed to do anything but destroy the disc .... but I have a hdd with a clear cd cover fixed in place of the tin cover - It runs win 98 perfectly ... and has done for the best part of a year now (I use it on my courses .... looks realy cool! and certainly impresses the delegates)