Twitter broken but just on MAC

Working one day and not the next. When I try to open twitter on my mac I get the message 'could not authenticate you'. No problem on iPad or iPhone and a Twitter add-on to Lightroom on my mac also still works.

I thought of deleting and reinstalling but having read elsewhere on an apple forum deleting can be an issue also (as in not fully deleting therefore unable to reinstall as the mac thinks it is already there!)

thanks in advance

Martyn