One forgotten or mistyped include, definition or declaration can generate many errors. The following links provide information and examples of how the errors can be generated.
Have a look at this MSDN page for LNK2019.
Here is a complete list of linker errors.
You can also perform a google search for each error by entering LN2019 or LNK2001 or LNK1120 and there will be many links that may provide some insight to your issue.