Go to www.skype.com and click the login link at the top. Is your credentials still recognised?
Your account may have expired, or you may need to renew your suscription.
You may need to install the latest version of the Skype software. If your installed version is to old, it may no longer be supported.
Put your mouse over the "Get Skype" button and selelct your platform.
It may be worth your time to consult the support page at: https://support.skype.com/
Electro
April 2012