You left me planted!
Admittedly, in English this doesn't make any sense. That's often the problem with literal translations, so let's get to the bottom of this.
First things first. If you don't know, dejar means to leave, as in leaving something behind. Here are some examples.
If someone is giving you a ride:
Déjame en la esquina
Let me out at the corner
You forgot something:
Dejo mis llaves en casa.
I left my keys at home
You don't want to be bothered:
Déjame en paz
Leave me alone
Great, now let's get back to our expression.
In this case, ¡Me dejaste plantado! is an idiomatic expression.
When you dejar alguien plantado that means you've stood them up.
No me dejes plantado
Don't stand me up
¡Me dejaste plantado!
You stood me up
This expression can be pretty useful and is nice to have in your toolbox, especially if one of your friends isn't exactly the reliable type.
¡Hasta la próxima!