Ok - things I have tried.
I do not have an ethernet cable. I am a guest here at place and people here out in remote France do not have much technical ideas either. The router box is located in another building with booster next door. The owners guests normally just get wifi if they are microsoft based but with apple they have found they needed a $ in front of this. They did not have this info at hand when I first tried to join wifi with my apple yesterday but I have never had a problem joining network before. My husband's laptop joined straight away (he has same apple laptop as mine). Yes there is a User name to network and then one enters password with dollar sign in front of it put into laptop.
What i have tried to do to fix problem. I tried restarting mine several times as directed... to make changes. Tried doing a PRAM reboot which I saw someone online had done and it cleared it. (pressed option, command, p and r keys altogether just at beginning of restart).... no this did not work. Tried entering details from my husband's network into mine but could not get router details. So I am at a loss as to what to do next. Would love someone to fix this problem for me. I am thinking maybe I need to go to McDonald's somewhere or some other place with wifi to try and get in that way and may clear it ....
In addition to Linc Davis suggestion also try the following:
See KB Articles: http://support.apple.com/kb/HT1401 AirPort troubleshooting guide
http://support.apple.com/kb/HT2712 Using network locations in Mac OS X
http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting
Also, run the Airport Utility app which is located inside the Utilities folder.
If using Apple's Airport, please re-post over in one of the AirPort Forums.