If you want to get the iPad home button on your iPad screen for any reason, you can turn it on via the iPad settings. You may want to add this home button on the iPad screen if you want more options ...
If your iPad says it's disabled, you'll need to connect it to iTunes or Finder to restore it. The exact steps will depend on which iPad model you own.
Anyone else have this happen? iPad 2, about a year old (although still within warranty, thankfully). Button would occasionally not register clicks, but it got a lot worse suddenly so I took it in.