I have applied more than 5volts into my digital pin 8 and now it does not work, what can I do?
Use a different pin? You will may find the part is running hot, and will ultimately fail.
Pick up another bootloaded part, or a blank Atmega328P-PU and use your still-functional arduino to install a bootloader.
basicaly
get a new board,
any 'bug' u have from now with this one, u are going to be wondering is it the board, my wirring or my code.
I suggest these guys
http://ruggedcircuits.com/html/ruggeduino.html
costs a few dollars more , but just about impossible to kill !!!
No I am not in any way related to these guys, I just like their product
Its saved my students a good few times.
Well indeed the chip may be on its way out - if you have a DIP socketed Arduino get a spare
bootloader-programmed chip or two (they really should come with a spare I think)
If not then put a sticker on the board to remind you - at least pin 8 has no special functions
like PWM or SPI or I2C [ assuming Uno ]. But always be ready to suspect the chip as the
damage could affect other parts of the chip or send it into latchup (suddenly gets very hot).
what do you mean bootloader-programmed chip? I get a new Atmega328P-PU?
Yes - one with a bootloader installed already, such as:
http://www.nkcelectronics.com/arduino-uno-ready-avr-atmega328p-microcontrol328.html
thanks
If you apply voltages like that to a pin, at the least put 2 diodes from the pin to Vcc and Gnd.
That will only shortcircuit your input device in case of overvoltage and the Arduino will survive 
if you dont want to break the inputdevice use a 1k resistor in series.