If you have a problem with Bluetooth, disconnect and remove from the Bluetooth device your phone and unplug the Bluetooth device power (if its a car, turn off the car completely, open and close the driving door, and wait 5min... if you have something to tell you that the system is off then wait to indicate that the infotainment system is out of power).
Then remove the Bluetooth device from your phone.
Then press and hold the power button and wait for the phone force restart (don't do a shutdown, the phone does like Windows 10 on PC where it partially hibernates).
Now, wait for your phone to full boot, once in the start screen wait a moment, as it continues to load everything, and then connect to the Bluetooth device doing a pair as if it was a new device with your Bluetooth device. If that fails, try using a pairing code method.
This should work. Now, keep in mind that it won't fix the performance issues, or other issues, but you'll be able to connect again.
Hope this helps.