ATTiny85 not working after burning bootloader
I suspect you have set the controller's fuses to use an external clock source as it is impossible to get the internal oscillator to run at 20MHz. You can blow new life into your controller by connecting a clock signal to pin 2 CLKI and try to reset the clock to 8MHz internal RC clock.

There is actually a 'fixed' version of ArduinoISP that has a clock output on digital pin 9. I didn't test this clock output myself, but it is worth to try. So in addition to the connections you already have between Arduino and ATtiny, connect Arduino digital pin 9 to ATtiny85 pin 2 (CLKI). Then try to program the 8MHz internal RC clock.

If you check the datasheet chapter 6.2 'Clock sources' for details.

An alternative method is using a High Voltage programmer, which can easily be improvised using an Arduino.

BTW: A similar thing will happen when you select a RC clock source &lt; 1MHz, your controller clock in that case will run too slow for the standard ArduinoISP software.