When using AAA authentication you can specify multiple authentication methods. This means if your primary authentication method is Tacacs+ and the server goes down it will autimactically revert to the secondary authentication method, that being whatever you have defined.
aaa authentication ppp default tacacs+ local
This will force ppp authentications to use tacacs+ as the default and if the server is unavailable then it will prompt for authentication using the local usernames and passwords.
Hope this helps.