Attention! You can select only ONE category in Wordpress

I have a wordpress site and today I just found that whenever I post something I cannot select more than one category.
I have not changed anything from long and earlier it was good .

I can not select multiple category.
The it just says
Attention! You can select only ONE category


Any person know any solution ?

you may have installed "Article Directory" plugin.
It has feature which disallow you posting a article in more than one category.
Goto Settings --> Article directory and disable "Warn if posting more than one category" .