You have multiple possible issues here. First off - are there any access lists defined on this router? If so, are you sure they are configured correctly to allow the desired traffic?
Second, like everyone has mentioned you have a possible routing issue. When you can't ping, are you trying every interface on the router or one in particular? If you are only trying one, what happens when you try some of the others? Are all of the networks part of the same major IP network or are you using multiple subnet ranges? What are you using for routing i.e. RIP, EIGRP, OSPF, static routes, etc.?