Try to restart your phone by holding sown the sleep button until you get the power off slider on your screen. Let it shut down and then restart it by pushing and holding the sleep button again until the apple appears. If that doesn't work try syncing to iTunes.
Have you tried doing a Restart? It usually is enough to sort everything out. To Restart, hold down the Sleep/Wake button, then wait for the Slide to power off message to appear. Then slide to turn off. After it has turned of, wait a couple of seconds and then press and hold the Sleep/Wake button again until the Apple logo appears.
If that doesn't work, try doing a Reset. To manually reset, press and hold both the Home and Sleep/Wake button for at least 10 seconds or until the Apple logo appears.
If that doesn't work either, you'll have to do a Full Restore. This will put it back to its factory state. You'll need to do this in iTunes. Connect your iOS device to your computer, and click on its name in the Devices list on the left-hand panel in iTunes. Click on Restore. All your info., such as contacts and settings will be backed up, so you can reinstate them to your device after the restore process has finished.
FYI, if you have a jailbroken device, you'll have to jailbreak it again after doing a Full Restore.
This thing happened to me. I recently installed a theme on my ip4, just to find out that the calendar icon had disappeared. I went through some of the forums suggesting what might have happened and the possible solutions but with no luck. Then the thought to remove the latest installed theme crossed me mind :) and voila! Got the calendar icon back, as good as new. See if you've installed anything (from cydia perhaps), which might be conflicting with the cal icon. Cheers mate.