Many Gmail subscribers are again getting irked today as the "Contacts" of the email service is not accessible. Although you can still login like usual, you cannot readily access the email addresses of the people who are listed in your address list. Likewise, GTalk is also having problems and this is why you cannot chat too.
I tried to email successfully but I have to manually extract the email address of the receiver. The official Gmail blog has yet to reveal what is causing this glitch. For now, you might just want to extend a little bit of effort trying to figure out the email addresses of your friends. Good luck!