I had exactly the same thing happen on my first grow. I did exactly what psteve suggests. It was actually beneficial in that when I bent it over, the bottom buds recieved much more light However it wasn't ideal. Next time top your plant early to stop it getting so tall. You'll also get more main colas and a better overall yield. Check that the strain is okay bieng topped tho... theres a few finiky strains that won't yield more if topped. Chronic is one I think.