Tragic things always happened before you know it, for example, you just cannot remember the passcode to access your iPhone to make a phone call. After attempting to enter the passcode as you can memorize to no avail for a few time, your iPhone will be disabled for long interval. After too many unsuccessful attempts, you won't be able to try again until you connect it to the computer with which you last synced it, if it has been synced to a computer.
If you can remember the right passcode during the interval, that would be great happy ending, you can leave this post alone.
If you cannot remember the passcode, you need to restore your device using the computer with which you last synced it with iTunes or iCloud. By doing so, you can reset your passcode and resync the data from the device or restore from a backup. If you restore on a different computer that was never synced with the device, there is possibility to unlock the device and remove the passcode, but all your data will be overwritten and erased. The restore process may wipe everything out. Even the Apple representative cannot unlock your iPhone without restoring your iPhone.
To learn more details, please refer to our guide about How to recover contacts from iPhone if I forget the pass code?