First step: Clear the browser cache and cookies (be sure to deselect any options to "save" some data, that may very well be the data you need to clear - note you will have to sign into accounts that are usually automatically signed in).

 

Second step: Be sure YouTube was not inadvertantly added to a list of sites that are not permitted to set cookies. For IE it would be in Internet Options, Privacy tab, in the Settings section, the Sites button.