All aspx pages stop working - Only Content controls are allowed directly in a content pag

I am no SP developer but hope someone would have insight to point me in the correct direction. These pages uses masterpages. Nothing has been changes, based on the file change/modified date. If I detach from page layout and move som <asp:content> tags, pages start working again. If I attach back to page layout, it breaks my pages. Any ideas where to look for the issue?