I encountered this problem on 2 differenc PCs already. After a fresh install of Qt (with online installer) on a PC where it has never been installed before, everything works fine, but once I update Qt (and Qt Creator, e.g. from 4.12.2 to 4.12.3) Qt Creator window does not show after launch. In the task manager I see the "qtcreator" process but that's all. I have to terminate it in the task manager. Even completely uninstalling Qt and installing it again does not fix it, I simply can't get it working, I also tryed to delete "USER\AppData\Roaming\QtProject\qtcreator" folder but it did not help.
Strange thing is that if I create a new user on that Windows PC, Qt Creator starts completely fine on that new user.