I have a MacBook Air that is no longer able to connect to TC for TM backup. I also cannot mount TC from the Finder. The MBA does, however, successfully use TC for wifi. Also, other computers in the house do connect to TC in Finder and for TM backup..
Please take a look at these excellent Time Machine troubleshooting procedures from one of our Discussions members Pondini. They should give you some things to try to see if you can get this resolved quickly: Apple Discussions: Time Machine - TROUBLESHOOTING