When I tap on Pinterest app I get a black screen. Where has my Pinterest gone?

Try this:

Quit all apps: Double tap the Home button and swipe the app preview page up

and Reset: Hold the Sleep/Wake and Home buttons and don’t let go until the screen goes dark and the Apple logo appears (no data will be lost)

Try a reset: Simultaneously hold down the Home and On buttons until the device shuts down. Ignore the off slider if it appears. Once shut down is complete, if it doesn't restart on it own, turn the device back on using the On button. In some cases it also helps to double click the Home button and close all apps BEFORE doing the reset.