Hey guys! Sometimes the smack will work, since a hard jolt is most likely what caused the issue in the first place. If that doesn't work, you'll need to open up the iPad and gain access to the LCD cable. Most likely that cable is loose and needs to be reinserted into the ZIF connector.
Then if that doesn't solve your problem you will need a new LCD. I can't really give you exact links to help you with the repair until I know what model of iPad you have. There is a number on the back that starts with A and then has 4 digits after that. It should say either A1395, A1396 or A1397 if it is an iPad 2.
You can find out what model you have on this page on Apple's website.
Just let me know what model iPad you have (iPad 2, iPad 3, etc.) and I'll help you figure it out.