Stuck on headphone mode

You can arrange a repair at Apple which will replace the phone with a new or remanufactured equivalent for the price in "Other damage"

https://support.apple.com/iphone/repair/service

 If your phone is too old to be listed then find a competent third party repair shop on the high street.

Check bluetooth settings- you may be connected to a nearby device and it will still show headphones graphic.

After that its a repair and if you have a phone old enough to have a 3.5 jack then repair is not financially viable.

I mean it will cost $300+ to repair.