Or, another cause could be that you have multiple .sln files in the project folder. Or, the GUIDs of the projects in the .sln file do not match the defaults. For example, default Kentico 8.2 web site project type should have these values in the .sln file:
Project("{E24C65DC-7377-472B-9ABA-BC803B73C61A}") = "CMS", "CMS\", "{420F4FED-2F28-4BE5-8CEA-9A6B9A57558B}"