Comments don't work

Hi

I don't know how long it has been like this, but I get a 500 internal error code when I try to post a comment, when I refresh it goes to a 405 error.

I updated wordpress last night, not sure if that has anything to do with it. Still on 3.7.3 as I haven't had time to update.

Any ideas what it could be ? Or are there loads,

My other site is on the previous wordpress version and works fine, could it be wordpress I wonder.

Thanks