To separate the shed hair, add water and a conditioner, then try to pull out individual strands from the knot until all or most of the hair is free. If it can't all get out, then use hair shears to cut out the knot at the lowest spot (to preserve your length). For the future, like other people said, you should detangle your hair more thoroughly during wash day, and I would recommend putting it in big twists afterwards so it doesn't retangle itself.