Why did i wake up with bright green tongue?

Something external: Check to see what you may have eaten or drank at bedtime last night, including toothpaste or mouthwash. These can have an effect, especially if you are a mouth breather and have dried it to your tongue. If there is no obvious cause of the "green", you should have your dentist check it out.

More info needed: This is one of those circumstances which requires a lot more information before even knowing who to refer you to. What is your physical condition - are you generally healthy? Do you have any systemic conditions, such as diabetes or an auto-immune condition? Do you have a special diet? What other symptoms did you notice along with this? Did it go away or is it still bright green?