You can restart your iPhone by quickly pressing on the volume up button and then the volume down button once and holding the power button until an Apple logo appears. This will restart your iPhone and possibly fix the issue.
If not you can also turn it off and on again the classic way: Restart your iPhone - Apple Support
If none of these work try deleting the device from Bluetooth and turning the Bluetooth off for a couple of second. Then turn it back on and try to connect it again.
If the issue persists you can scroll down to the 'Music' App tab in the settings app. After clicking on the App settings you can scroll down and press on EQ. Try using the "off" EQ. If it does not work you can also try the "Late Night" EQ. For some reason it always works out for me. After pressing on the right EQ setting you can try to play some music on the speaker. I hope it works. If you encounter any other problems, feel free to write in this chat. Best Regards