After installing network, menu items disappeared and no access to members, groups, etc.

I installed network and I the menu items of groups, members (all but the home and about menu) dissapeared. When I try to access them otherwise I get the messate:

"Sorry, we can't find the content you're looking for at this URL. Please try selecting a menu item from above or to the side of this message to get where you'd like to go."

I get this message also when I try to access a post

Anyidea what is goin on? And how to fix it?

Thank you