I would suggest first:
Do HARD RESET:
Take out battery
Hold volume +
Insert battery while holding volume +
Your phone will boot into recovery mode
Use volume + - to move up and down
Use power to confirm action
Your have to select action Wipe Cache/ Wipe all data
Second of first didn't work:
You need to re-firmware the phone
FLASHING INCOMPATIBLE FIRMWARE WILL TURN YOUR PHONE INTO A BRICK SO DO IT AS YOUR OWN RISK
_Get Odin.exe
_Get correct firmware from Tsar3000
(to get the most correct and compatible firmware get the model that match your country + original carrier)
Finally good luck: if are just amateur don't try second method brig to a phone shop instead to save your trouble