Had same problem with my mobile. Bought a new battery and charger but no improvement.
Just buy an inexpensive mobile from the UK. Take it into Turkcell or Avea along with your Passport, previous contract and Residency Permit (if applicable) and your old mobile. You will then be able to retain your old SIM and number.
I did this on my return from the UK recently.
Good luck and as usual you will need plenty time and patience!