There is no generic "rollback script" for this sort of change that anyone can give you.
You need to take the soure of the procedure, and change it to remove the change you made last week.
It is a very good idea to have some form of version control system to hold all stored procs, triggers, etc etc. We use Subversion - but there are many others. I fyou do not have that in place you can always script ethe procedure as it is now from the database.