wifi won't connect anywhere but home

I have seen many threads on this and tried different things, but I can't say that I really know what I'm doing and what is the best thing to do.

My problem: My wifi works just fine when I'm at home (I have a macbook pro and I'm having the same problem even on my iphone) but anywhere else I try to connect to a wireless network, I get an exclamation mark at the top. When I plug the ethernet in, the exclamation mark goes away and it shows a signal, however, defeating the purpose of wireless as when I unplug the ethernet the signal goes away. I used to be able to connect to my 2 other networks (work and friend's house) fine and it just started suddenly. There is no password needed for the work network and I haven't changed the password or anything for my friend's network. Same thing on my iphone, it will find my home network, but not the other two.

I don't have access to the router at work, but could play around with my friend's router. I will continue to try out different things..but any suggestions much appreciated..will post whatever solution I find.

When you click on your wireless icon does it show the network you want to join and did you click on that network ?

Yes, when I click on the airport icon at the top, it shows my networks in the list. It shows there is a signal and there is a check mark by my network - with the ethernet line plugged in and when its not.

It always has a self addressed IP address (except at home) but the airport icon shows a signal there only when the ethernet is plugged in. It changes to an exclamation after I take the ethernet line out, even though when I click on the icon and the network in the list shows there is a signal. I've tried turning airport off and back on, and deleted the network and added it back.

I've had connection problems at home a couple weeks ago with my imac and macbook after a bad storm that knocked out the cable altogether, but I was able to fix that easy enough by restarting everything and I changed one setting in the imac. I thought the problem was then in the router and/or modem (or just the cable company) and was fixed. But ever since then no other network has worked on the macbook except for home. And I'm stumped because I didn't change a thing on the macbook.

System Preferences > Network > Ask to join new networks (is the box checked) ?