Are you sure you have the latest samsung drivers for your phone? if not download it from the internet.
And then follow the below steps:
Go to download mode(vol up & down + power buttons)
wait for sometime ,you will come to a warning screen on your phone,press the vol up button,wait again
now check on odin on your computer,odin should have communication with your phone.
And you should have necessary files to unbrick ,if not do the following:
Download the latest firmware>use splitfus to obtain the PDA,CSC and MODEM/PHONE files
you will also need bootloader file and .pit file
Use odin to unbrick.
Easy when done right