mrburnette:
The Mega2560 board has the external crystal/resonator... Therefore this cannot be the issue with the main uC... Either internal OR external clock should work. The ACRISPmkII is a fine programmer, so either you got the fuses really tangled to the point of requiring an HV programmer or something else is weird. Perhaps one of the guru members will stop by and offer an idea.Ray
It is indeed possible to accidentally set the 2560 fuses to "external clock" which will render the board useless unless an external clock can be provided in order to "run" the chip so that it can be re-flashed correctly.
I've already made that mistake and luckily I was able to save the board by "borrowing" the oscillator from the 16U2 (USB interface) chip.
For removable micros like the 328P, these chips can be rescued either by using the "high voltage programming" method along with the appropriate wiring or shield (such as this: HV Rescue Shield 2 | MightyOhm) which, by the way, works very nicely, or else use the Adafruit programmer kit (Standalone AVR ISP Programmer Shield Kit - includes blank chip! : ID 462 : Adafruit Industries, Unique & fun DIY electronics and kits) which very cleverly uses the host Arduino board to generate an 8mhz clock for the micro, allowing it to be programmed even if the clock source is set to "external".