The most common mistakes using LEDs with a '51 are the following:

1) omitting series resistor with the led - typical result: a blown '51 or a blown '51 port pin.
2) connecting LED and series resistot between port pin and ground.
3) using a LED that require more than the few mA that a '51 pin can deliver when low.

Since you do not post a schematic, check the above.

Erik