ANSWER:

There is no sin on you because of forgeting the Quran. 

However, you should keep revising those surahs in order to be elevated in jannah. Why would you want to waste some much of time and efforts that you put into it.

I had memorized few juzz of the holy Quran and then had to discontinue. I was told not to forget the memorized parts by my ustaads but as years went by I have forgotten most of what I had memorized. What is the sin of forgetting and what effects does this have in my life . I do intend to learn it again . InshaAllah.