Old-fashioned desks and their drawers used to have matching grooves in their sides to facilitate the drawing in and out of the drawer from the desk. This system had no stop or brake, however, to prevent the drawer from falling out of the desk and onto the puller's foot when pulled too far. Newer desks with runners have a safety mechanism on the upper backside of the drawer to prevent this from happening. It is fairly easy to disengage this safety mechanism so you can remove the drawer from the desk.
Pull up and out on the drawer to disengage the drawer's rollers from the rollers on the runner. Pull the drawer the full way out until it stops.
Support the whole drawer front with one hand. Do not merely hang on to the drawer knob or handle, as the drawer could slip and accidentally fall on your leg or foot during the removal process.
Locate the safety mechanism, usually a spring, near the upper backside of the drawer. There may be only one on the right side, or one on each side.
Press a thin, flat card such as a credit card against the spring on the right side of the drawer to disengage it. Gently wiggle that side of the drawer free. Pull the drawer free slowly.
If the drawer will not completely pull free of the desk, look for a safety spring on the upper left backside of the desk drawer. Depress it with a flat card. Pull the drawer straight out from the front, using both hands.
You can try to use a screwdriver to depress the safety spring; however, there is usually not enough space between the outside of the drawer and the inside wall of the desk to fit a screwdriver.
Modern desk drawers are deceptively heavy; use caution when fully removing a drawer from its desk to prevent it from falling out and hurting you.