Im assuming you have atleast a little experience customizing roms when you messed with your stock rom in the first place.
1. Turn phone off remove battery for one minute (some custom rom with custrom kernel like kuro has undervoltage features which retain charge longer.. well thats my observation)
2. Reinsert battery, boot to recovery (POWER+HOME+VOLUME UP) wait untill galaxy logo appears then release.
3. wipe DATA and CACHE partition. Its the third and fourth option in stock recovery mode.
4. If you have CWM recovery go to advance and wipe dalvik cache also.
4. Reboot
Didn't work?
1.Repeat steps 1 to 3
2. Reflash your custom rom
3.Reboot
Still not working?
1. If you are in stock kernel and did a CWM backup of your stock rom follow this if not go to last resort flash via Odin
2. Boot to recovery go to CWM recovery and restore backup
What still stuck?.....
1. You will need Odin here. go to sir Doky's page for flashable firmware package.
2. Download his firmware from your region, he has it all, also his Odin app.
3. Extract the package( you will get 3 .tar files) and install odin to your Pc. Please tell me you didnt throw away your samsung driver cd, you know the one in the box? install the drivers for your usb if the odin app doesnt have the drivers when you enter download mode.
4. I hope you didnt flash an Uber kernel like merruk kernel in which case you will need a TOTORO pit file (ask around for it) put it in also and click repartition in odin. If not ignore this step.
5. You are now ready to install fresh stock rom.
6. Go to sir Doky's page for instruction
7. If successful you will need to dial *#272*your IMEI# then click your country's sales code(google it)
8. Reboot
Hope this helps you buddy