hacking into my router

[CaptsCove]

We have a small business and offer free wifi to our customers. We have a netgear router and have service through our local cable company. Yesterday we started getting complaints that when someone tried to go online a message popped up "$2 shots tonight"!! Obviously someone hacked (if that is possible) and I need to figure out how to stop it! The actual router is up in the rafters, so it is impossible for them to physically do anything to it. So I am assuming there is a way to do it online!? I am not very computer savvy and would have no way to figure out how! It is not password protected, because it is supposed to be free to our customers!

Any help from anyone would be appreciated!