phone on full volume but sound doesnt work, wont let me change it

this keeps happening sometimes when im in the car. ill have my headphones on yeah, and then the sound just stops working. it shows it at full volume but i cannot turn it down when i try. its still like that when i disconnect my headphones too. no warning, no instructions of how to turn it off. i would appriciate if anyone knows how to fix it, please tell me as it actually dives me mad