There is a bug in Calendar that causes this on some devices. It is caused by the April 1st entry. If it is entered as an all day event reduce it to an hourly one, or if that doesn't work delete the entry for now or place as a note on March 31.
As for getting it going again, double press the Home button to expose the task bar at the bottom. Press and hold any app icon until they all jiggle, find the calendar app and tap the minus sign to force it to close. If this doesn't work do a reset/reboot. Hold the power/sleep button and the Home button together and continue to hold until the Apple logo appears, your device will start up after a longer period than normal.