First question: Did you have your neighbor's permission to use his wireless connection? If not, then that's probably why.
Consider that if you didn't have permission you could get into real trouble for piggybacking on someone else's wifi signal, and sometimes even if it's unsecured.
Hopefully this isn't the case...
If you do have permission, ask him what happened. It's possible he might have changed a setting on his AP that would have affected your access.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1