Flashing stock rom.
Bro, are you trying to flash back to stock Samsung ROM? if so just google the Model number of your phone followed by stock rom
EG: GT-i9500 Stock ROM.
Okay then Put it onto your SD Card (extSD or intSD) and flash the rom the way you normally would
But next time Do this then you can revert instantly:
1.)Download a APPLICATION BACKUP APPLICATION ie:Titanium Backup
NB: DO THIS WHILST ON STOCK ROM!
Enter recovery Mode (im using CWM in this instance)
2.)Scroll down to "backup and restore"
3.)Enter and select "backup"
NOW YOUR ROM IS BACKED UP (WITHOUT APPS (HENCE WHY WE DOWNLOADED A BACKUP UTILITY))
4.)Next time just backup and restore from that backup you made and install the apps you had using your backup utility
Enjoy your day, Hope this helped!
STICKxaH Loves you and xDa