So i was making a mod right and basically ive accidentally deleted some stuff needed for a quest and only realised a couple of days after. My question: is there any way of resetting cells back to their original state or maybe copy an unchanged cell from one plugin into the other plugin which has altered it?

Follow these instructions for the Construction Set (which are still valid in the Creation Kit):
http://cs.elderscrolls.com/index.php/Mod_Cleaning_Tutorial#Use_The_Details.2C_Luke