Why is my wrt54g router saying destination host unreachable and the power light is flashing?

i ping my router and it sends and receives 4 packets but beside of that it says destination host unreachable and the power light is flashing i was given the router by a friend and they said it works.My operating system is vista and the router is a linksys wrt54g v8.i can't access the router or the internet.my modem works fine when i direct connect it.any ideas at all feel free to post.