When the computer is turned on, does it make all the usual sounds that indicate it is powering on? (fan noises, etc?) If not, go to (b). If the computer makes all the sounds of powering on, but you don't see any display, go to (a).
(a) Check that the video cable is seated in the correct video connector on your Tower, and that the other end is also connected properly to your monitor. If this is as it should be, then check that your graphics card hasn't been dislodged. (Don't touch the components -just the edges) If this doesn't make any difference then try (b).
(b) You will need to take a look at whether your new hard disk is causing the problem - take it out temporarily and see if the computer starts normally. If the computer starts normally then try fitting the drive again being sure of the connections. If the computer won't start with the drive fitted, and you are sure of the connections and drive jumper settings, then change the drive.