Have you verified that BIOS is correctly seeing the new drive? Did you use
the software that came with the drive to properly set up the new drive? If
you can't get even the monitor to come up, you probably either knocked
something loose while you were in the case, or something got nailed with a
bunch of static. Make sure every cable in the computer is firmly seated.
Clear CMOS (refer to your manual for the proceedure.) See if it boots up.
If not, disconnect ALL drives - see if the monitor comes up and you can
enter BIOS. If it will boot into BIOS without the drives, but not with, the
new drive is probably not correctly configured (i.e. you probably do not
have the jumper correctly set.)