The wooden solution is to add the path to the Qt DLL in the PATH.
If you use Qt Visual Studio Add-in , then this work should be done by it. Moreover, it does this implicitly, it literally processes the start-up debugging event and adds the path to the DLL to its copy of the PATH process (it seems that with the usual non-debugging start it does the same). Accordingly, you must specify this path in the Qt project settings. This is done in order to be able to compile projects for different versions of Qt. So, I remember, I was faced with the fact that the Addin in this place was buggy and prescribed the way through time.
Found one and two . Try, as on the first link the person writes, to rebuild addin.
In general, it is very difficult to judge why your program on your machine does not find the DLL. You wrote what used to work, see what you did with the system during this period of time. And yet, in accordance with the MSDN check whether you have everything configured correctly.