I found this from google.
Hope it helps you out...
Here are the troubleshooting steps that we typically share:
1) Make sure you have enough available space (i.e. no low space icon)
2) Power off your phone, power it back on
3) If you still have pending downloads, cancel all outstanding/pending downloads. Don’t worry, once you’ve paid for app you can install it as many times.
4) If pending downloads will not cancel, uninstall the application. Once again, don’t’ worry, you can install each application as many times as you’d like once you’ve purchased it.
5) Re-download your app(s)