I programmed the RSTDISBL fuse and now I am not able to reprogram my AVR, what is wrong?
Please note that your AVR needs an external RESET source for the ISP interface to work.
When you programmed the RSTDISBL fuse you disabled the ISP interface at the same time.
To reenable the ISP interface you need to use High Voltage Programming to unprogram the RSTDISBL fuse.
To do this you need an STK500. The procedure and connections for High Voltage Programming is located in the AVR Studio Helpfiles:
AVR Tools on-line help --> STK 500 User's Guide --> Hardware description --> High Voltage Programming.