Editing or creating a linked dll file

I have a simple exe file linked to a dll file, and it uses one function from it - GetHour (which simply returns the current time).
I want to change the dll file and insert my own function so that everytime the exe runs, a new file with some text in it will be created. Unforunately, I'm having trouble doing so.
I can't open the dll file with DotPeek, probably because it's not .NET (I don't fully understand what that means).
I tried writing a new dll file, with its own GetHour function, but failed due to the following error:

The application was unable to start correctly (0xc000007b)

I don't really know how to deal with both my problems, and would really appreciate some help. Thanks in advance.

.NET (I don't fully understand what that means).

On windows, you have two "types" of DLLs. You have "normal" DLLs, that contain native code (=Machine code) and .NET DLLs that contain the IL that was emitted by the C#-compiler (for example). See here for details

DLL files are PE files, so they follow a special fileformat and if the DLL you created doesn't match, you will get this error. Editing a PE file in a hexeditor is quite difficult and errorprone.

There are tools like PE-Explorer, that will help you to fulfill your task, but they often require a good knowledge about the PE File format.