I accidently applied around 5V to the output pin of the 555 timer.The output is zero when I ground the trigger, and an LED connected between 5V and pin 7 will not respond when 5V is applied to the threshold. Pin 8 is also connected to 5V. Is it damaged? Could it be something else?
If you have another working chip you can set up a circuit with that chip (e.g. an LED blinker). If the circuit behaves as expected with that chip, then swap in the one you think is damaged and see if the circuit still behaves. If it doesn't then the chip is dead.