I had never experienced this until today it happened to me for the very first time in almost 16 months of using my iPad1. I simply tapped and held down on the waiting icon until the X popped up - deleted the app - and then downloaded the app again. I had 6 updates and I had to delete the first one and then reinstall the app and the the other 5 downloaded OK. That is not the most practical way of updating apps if you have data that you do not want to lose - so here are the tips that I always recommend.
Check Restrictions - Settings>General>Restrictions>On. If they are turned on you may have disabled downloading apps.
Check to see if you have a stalled download in iTunes - a song that did not finish downloading or a podcast perhaps. Finish those first if there is one in there.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.
Try signing out of your account and restart your iPad. Go to Settings>Store>Apple ID and tap the ID and sign out. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button. Go back to Settings>Store> Apple ID and sign in again.
If you are getting the "waiting" icons - Tap on one of the "waiting" icons and see if you can get one of them to pause - then tap again to see if they will resume downloading.