No Input Source Selected ?

I keep getting this message when I try to record. The thing is, I DID select an input source...

Garage band was working fine with my laptop mic until I plugged in a microphone. I made a recording, then unplugged it to go back to using my computers mic. Then I keep getting this error message. I am going to preferences, audio, and selecting built in microphone yet it keeps telling me "no input source selected" and wont let me record anything, If I plug the external mic back in, I still get the error message.

I've read that garage band is supposedly the most simple way to record music, but to me it couldn't be any less intuitive. If I press record, it should record. There shouldn't be anything else I have to do. Grr. By the way, I have tried closing out of garage band and re-opening it in case it was frozen or something of that nature but I still get the error message.

In any case, while I am on here I have another question. Once I figure out how to record again (if I ever do) I need to know how I can play something and then loop it, and then play over that loop. Like lets say I want to play a 15 second drum beat (not a computer simulated one, but a drum beat on my actual physical drum set) and then loop that one beat over and over for a ten minute song while playing guitar or whatever over it. I get how to select a section of a track and make it play on a loop, but then when I try to record something it will only let me record for that 15 second interval or however long the first recording (looped) section was. I need to have the loop play indefinitely while I play a new track over it. Does that make sense? How can I do this?

If this doesn't start to make more sense to me soon I'm about to give up and go back to my tascam recorder and cassette tapes. They may not sound as well, but at least I get how to use them to make a simple recording!