Press and hold these four buttons simultaneously: (star) + 3 + Green/Call button + Power button.
This method fails on newer Symbian firmware that has patched the master code vulnerability. In that case, use Method 2. How to Hard Reset NOKIA E52
This is the "deep" hard reset used when the phone is frozen or you can't reach the menu. your Nokia E52. Press and hold these four buttons simultaneously: (star)
A prompt will appear asking for confirmation to "Erase and restore factory settings." How to Hard Reset NOKIA E52