My lens fell out of my canon rebel k2 and I don't know what to do how do I reassemble it?

Consult your camera manual for specific instructions.

Carefully blow any dirt off the rear lens element (use an air can).

Note: NEVER USE AN AIR CAN TO BLOW INTO THE CAMERA BODY!
You can easily damage the sensor or mirror with the high-speed air jet. Generally, there will be a red dot on the lens and another on the camera body. Match up the dots, insert the lens into the body, and turn it into position. You should be able to tell the direction to turn the lens by the location of the writing on the lens. The rotation is usually clockwise, as you look at the front of the camera. Do not force it. If it is fit properly, it should turn with little effort.