its called "writers block/ or writers slump..."
I do it all the time... I can think of some shit
when Im out havin fun, but when i sit down
to write some shit, there are no lyrics for me
to write down...

solution- carry a pen and paper with you EVERYWHERE you go,
so when you get an idea... you can write that shit down...
after a week or two, when you feel like jotting some lyrics down,
just pull out that paper, look at your ideas, and go from there.
I've found it helps out a lot.... (not a bright Idea to use in text
battles though, cuz thats called pre-writing, use only in open mics, or for writing songs you plan to record.)...
~Ashy~