My data content type does not appear as an option on my 'Add content' page I have just created a new content type for my site, and wrote a few rules which act when an instance of that type is created.  I went to create a few test instances of my new content type and noticed that it did not appear on that 'Add content' screen (mysite/node/add.)  'No problem' I thought, 'I'll just go to mysite/node/add/new_type.' However, this failed to work too.  The page loaded as as the url I submitted, but it appeared to be a duplicate of mysite/node/add.  This is a problem because I can not add new content of this type.

I have a few other content type that do not appear on the mysite/node/add, but this was never problematic because I could still create new content of that type by visiting mysite/node/add/some_other_type.

I found that I can create new instances of this content type using Development: Generate Content (and the rules work just fine.)  Still, I have yet to figure out why I cant get to the creation page manually.
What could be causing this?

Edit ===================================

A newly added content type (just called test with no other changes or fields added) does not appear either (even with all permissions set to 'registered user').
It sounds like there is a problem with your menu system.

This is an (educated) guess but rebuilding your menus may help.

Install the devel module and then click on Rebuild menus: