My Girlfriend Disappears For Days And She Doesn't Call Me Or Answer Her Phone. Any Suggestion?

Face the possibility of ending relationship. There is the probability she may be cheating on you.....
She disappears for days...!? There is something very wrong with this picture. Have you asked her why and where she was? Was this work related? Weekend, weekdays.?...If she speaks about it, are you satisfied with her explanation? Is this behavior consistent? Is this behavior the cause of arguments between you two?
If you love and trust each other it is not all right to disappear for days without giving you, or anyone for that matter, a reason as to why.... There is no need to say much if this behavior repeats itself from time to time.
If I took off from work, suddenly, you better believe I will have to notify somebody....
Move on find somebody else.

Don't  believe or trust her

Well, dude in a Relationship if he/she lies to you. You have no choice but to believe.
But not answering the phone sounds terrible.
You must add the details, the day, before she left you, there must be a reason why.

Well, your comment suggests one thing .. A lack of honest communication. 

Therein lies your problem.

Why does she do that?  Did you fight and that is her way of getting back at you?