I know you this is a silly questions but I'm pertty sure you know how to replace the toner catridge and reset the counter on the new toner catridge. Click on link for example http://download.brother.com/welcome/doc100487/cv_hll5000d_us_eur_drum_insertion_a.pdf
You can also try to reinstall the driver on your computer that sometimes helps too.
click on the link for user manual on your model:
http://download.brother.com/welcome/doc100514/cv_dcpl5500dn_use_busr_a.pdf
I hope this information helps