|I guess, this is your router, we are talking about.|
Anyway, a ping is "hello are you there?". But if the system you're trying to ping is under heavy load, it will not give an answer to a ping, because it's not that necessary.
E.g.: If a router is under attack, it will stop responding to ping and tries to resist to the attack.
The original poster should always write the last response !!!
Let us know, if the problem is solved !!!