This is sort of a hard one to gauge.
You have to look at it this way:
Has this person emailed you before? Do they know your email address still?
How often is this person online, would there be better ways to establish contact, ect.
After going over that, if you'd still like to pursue willing this person to email you, then here is my suggestion:
Think of this person heavily. Think about yourself and this person together... some memory you share.
The first step is to put your vibe out there so that this person might pick it up and start thinking of you as well.
They may contact you, but it may not be via email. I'm guessing any medium would work, right?
Sometimes you just gotta take what you can get
And if they don't contact you... you could always take it from there.