Thanks for sharing your concern.
Brown junk on your teeth is nothing but the exposed dentin
,very rarely a blood clot too.
I would suggest you to visit your dentist
and get the vitality of tooth checked.
further it can be restored by composite restoration for esthetic reasons, as you have mentioned it is your front teeth.
I would also suggest you not to delay in this because it might cause further damage and the pulp might get damaged causing lots of pain.
hope it helps.