Sounds like it needs a new Charging flex as this is where the Back&Recent buttons are located on this model.
If you feel you are competent enough you can follow the guide below and attempt it yourself. Although this is risky as the LCD is held on with fairly strong adhesive and since its already cracked there is a high chance of completely breaking it upon removal.
[guide|123905]
Other than that you will likely have to take it to a local repair shop where they should be able to complete this repair.