unable to create a new thread in forum

Hi suddenly users are not able to post or create a new thread in forum. can you please help. i am getting the following error

Invalid Content Type
Either the content type you are trying to create does not exist, or you are not authorized to access the content type you have tried to create in the current context. 'Create' a content type by selecting from either the group menu or the 'Create content' menu instead

the website is a production website. please help asap. prior to this i i tried to create the organic groups and reved the same. i also updated the permissions link in post settings with no luck. please help.

i am using the advanced forum..