My iPhone 6s is stuck in headphones mode.?

If cleaning the audio jack of your phone (with a damp, lint-free cloth or compressed air) does not fix it, then you might need to bring your phone to a professional repairer.

That means there is either a bit of dirt or debris inside the headphone socket, making the phone behave as if the socket is being used, or one of the internal headphone contacts has broken with the same result.

You can try cleaning/dislodging debris by repeatedly inserting and removing a headphone plug, but if that fails the headphone socket module will probably need to be replaced and that requires extensive dismantling of the phone. If you’ve never done that before then there is a high chance that you will fracture the conductors on the ribbon cable which connects the phone screen to the motherboard: if that happens you’ll need a screen replacement as well.

try to restart it