There is a very good step by step wifi support section on the Ubuntu site at
Work carefully through this site following the instructions in order. This can fix many problems. You might want to do this with a wired connection available in case things go wrong.
Once you have done this very carefully see if the problem is solved.
If you problem has not been solved after working through this you can do the following that might help (it does for me on my netbook). Connect to the wired connection. Go to the software section and install WICD. Go to system settings-startup applications and add WICD as a start up. Uninstall network manager (this is very important). Reboot and see if this helps.