You might also have some luck in using the search tool. There are multiple threads already avialable on TCS login issues which may already contain some pointers and answers to your issue - some examples are:
If those threads don't help, are you able to describe in a bit more detail your specific confiuration, are you attached to a domain, are you using local TCS accounts, using groups, configured LDAP login?
And more details of what the sympoms actually are - is it not connecting at all, or are you being prompted to log in, but that fails, or something else?
Wayne
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