writers block come from lack of inspiration.
so in order to continue, first things first you cant force it, thats how you end up with those garbage lines that 90% of the music industry come out with.
secondly you need something that moves you, people will say listen to a song but it doesn't have to be that. it could be a powerful film, a person, a piece of art. or maybe just a story (fact or fiction) that you have an opinion on.
it helps if your inspiration is to do with your subject, ill assume your song is about a relationship/girl/boy (not trying to be narrow minded about RnB, just assuming forgive me haha). So if theres ever been a special someone in your life, you need to think about that person, romanticise them out of all proportion in your head, visualise things that never happened but seem like they could, completely exaggerate everything you loved about them or didn't love about them depending on the subject.
thatlll get the juices flowing.