Paintbrush tool

This may be a dumb questio but i am new to flash 8 for the most part. I made a square box with the line tool and when i went to fill it in by clicking on the paint brush tool and clicked in the middle of the square nothing happened. I was following a video tutorial and I did this two nights ago and it worked with no problem. I don’t know if i did something to the settings but when i went to do it again last night it won’t fill. Someone please help.