lose touch with (someone)
To "lose touch with" someone means to stop communicating with them. It's kind of the opposite of "keeping in touch with" someone.
When you "lose touch with" a person, you stop calling or e-mailing them, and you usually don't know what they've been doing recently.
To explain how long it's been since you contacted a person, use "for":
We lost touch with each other for a few years, but he called me last spring out of the blue and invited me out to lunch.