Why does Craigslist Pro+ fail to launch on an iPhone 4 running iOS 5?

Try double clicking the home button to see if it shows up as a recently used app. If it does, tap and hold your finger on it until it starts to wiggle, then tap the X to force the app to quit.

If it doesn't relaunch normally after that, try restarting your iPhone.

See this Apple doc for other ideas -> iOS: Troubleshooting applications purchased from the App Store