It's ok to feel sad , it's a part of transition. You aren't supposed to be all happy in the first day. I'm currently in college so I know what I'm saying.
Now what you can do: try to visit the campus! Go around it, go to the library, try to stop by a coffee shop and buy something.. so you can see what your college looks like and maybe you can meet some people :)
Trust me, in freshmen year people are DYING to meet other people, so trust me when I say that they'll be open to talk to you. So go ahead, explore, talk to people and have fun!
I promise you that you'll start adjusting bit by bit :)
And if you ever need someone to talk to about this, you can PM me