I cannot start my 2007 B&G Weedeater 21". I cannot pull the cord.

Hi,
First thing to do would be to pull the plug to see if you have fluid(oil and/or gas) in the cylinder hydraulically locking it. Barring that check under the machine for build up that may have dried and be holding the blade(yes, I fixed more than on by cleaning underneath) and remember the plug or air filter should always be pointed upwards.
Peace,
Clarke