Another common cause is the spark arrestor coming clogged up with soot after extended periods of use. The spark arrester is a metal screen, which is normally part of the chimney cap. The small holes allow smoke and hot gasses to pass through but stop large sparks or embers from passing through. If the system has not been swept for some time these small holes can become closed off by the soot build up. Rather than climbing up on the roof and hitting the spark arrester a few times with a hammer or the like there is another quick fix. Follow these simple steps:
Grab some aluminum foil and some tape. Be sure the tape you use will not damage the finish on the facing of your fireplace.
Using the aluminum foil, close off the top 6-inches of the fireplace by stretching it across the top of the firebox opening.
Tape the aluminum in place with the tape.
This should help the fire draft until the fire burns itself out.