It's not necessarily a mistake for you to replace your battery yourself, but there is unfortunately always a chance for something to go wrong, which you may have just found out.
Your phone has the older LCD technology for the display, which means it has a backlight. I'm not up to date on Motorola phones, but we've seen many cases with iPhones where plugging or unplugging the display while the battery was connected can result in blowing the backlight circuit, resulting in a black screen. Note that if you unplugged your battery ''before'' disconnecting the screen and made sure to plug the battery back in as the last step ''after'' reconnecting the screen, then chances are this wouldn't apply, as that's what usually blows the backlight circuit on an iPhone.
The way to tell if that's what's happened is to hold a flashlight up against the screen at a slight angle. If you can make out a picture that way, then your backlight is out. Usually that means replacing the screen, but there are some rare occasions when it has turned out to be a blown fuse or circuit on the motherboard. That's uncommon though, so if there is an image your best bet would be to pick up a new screen and hopefully that will cure what ails you.
So check out the backlight situation and let us know what you find. If the backlight is out then a screen replacement is your best bet, otherwise come on back and let us know and we'll see if we can figure out what to do next.