Check this apple doc out -> iOS: Wrong passcode results in red disabled screen
If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to 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, you will be able to unlock the device for use and remove the passcode, but your data will not be present. Refer to Updating and restoring iPhone, iPad and iPod touch software.
If you can't remember the passcode and don't have access to the other computer, you'll have to restore it and setu it up as a new phone.