Yep, sounds like you either:
a) inadvertently applied MAC filtering and didn't include your hw address to the list or
b) your neighbor caught you using his AP.

The first you should be able to fix easily by accessing the management tools from a box wired to it, The second you can probably fix if you're creative. I'd make sure I was able to kick the neighbors ass though before you try. Because I tell you what....If I saw repated attempts to connect to my ap from one of my neighbors, I'd be knocking on the door wanting to know why. I wouldn't be happy about it either.