Is it the 4 digit passcode that you entered incorrectly for multiple times?
If that is the case, unfortunately, the only way out is to restore the iphone to factory settings.
Do you have itunes installed in the computer?
Open itunes in the computer and connect the iphone to the computer. Now, on the iphone, press and hold the power button and the home button together until you see an apple logo. The very moment you see the apple, let go off the power button but keep on holding the home button for another 10-15 sec. Your computer should now give a message that it has detected an iphone in recovery mode and needs to be restored.
Restore the iphone now. After restoring, choose the latest backup. Let me know if you find any difficulty.