Please try to get your phone to get into download mode first. Check this video first!!!
https://www.youtube.com/watch?v=ZHC-8cDxeeI
You do that by pressing the 3 buttons(Home+power+ vol DOWN). You will get a message to press VOl UP to get into download mode.
Once in download mode, connect phone to PC running windows via USB cable, wait for the device to get detected(you will have to have installed the USB drivers first, in windows) by windows and then run odin.
in odin you select the .md5 file which is got by extracting the .zip file that you would have downloaded from sammobile or other website.
You do not need to root your phone to restore it back to original factory settings(like it was when you bought it). Once you have finished flashing the .md5 file, the phone will automatically reboot and you will realise that its back to stock condition.
Once the phone has been revived this way, proceed to boot into recovery(home+power+power UP) and then flash raw kernel r8 beta2, reboot to recovery, flash nightowl beta 2 and then the corresponding gapps and you are good to go
Hope this helps