How to reset ATmega328P fuses using USBasp programmer?
Its possible you may have disabled the SPIEN or the RSTDISBL fuses. The only way to check and fix is to use a High Voltage Programmer like the STK500, STK600 or AVR Dragon and see. I have the AVR Dragon and it saved me a few times.

Also, do a search for AVR Fuse Doctor. Its a homemade fuse repair device. Pretty cool. But save yourself some time and invest in a programmer/debugger that has HVPP (high voltage parallel programming) and HVSP (high voltage serial programming). You won't regret it.