Unless the phone has been rooted, you should not have been able to actually delete the app. While you could have made it not show on the phone home or other such screens, it should still be in the devices listed apps. Then all you would have to do is readd it to the home or similar screen.
If there was something else done to the device that removed it and or other system apps, then you would have to do a "Hard Reset" of the device to get the original apps back. In doing so the phone would be wiped first, so you would need to save your personal info, photos, music, contacts, etc., first to say a computer.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or...