Moving on...How to let go after 4 years together...He acts as if i never existed..?

I have been seeing this guy for 4 years...Out of the blue without notice he just drops me from his life like I never existed..No calls, no texts, no emails, no communication what so ever...it is going on 3 weeks since I have heard from him...I am heartbroken and was hoping maybe someone who has been in a similiar situation could help..someone who understands what i'm going through..I'm having a real hard time and the tears won't stop..I am left with no answers and no closure...