You need to uninstall yahoo messanger with revo,Then reinstall.
Download revo from below link.Click here
Then install and run revo then select yahoo messanger and uninstall.
Also remove all registry keys when it ask for remove.
Then download yahoo messanger from below link.Click here
Then install yahoo messanger in computer.
After uninstalling yahoo messanger go to c://programs files/yahoo/messanger then delete all folders of yahoo messanger.
IF after reinstallin still same issue then wait for 48 hours because yahoo sometimes block id for 48 hours.Then you can use it again.
Also make sure that no virus issue in computer and internet is working fine.Turn off your wndows firewall and antivirus firewall for checking.
Let me know if you need further assistance.
Thanks for using Fixya.