I had this problem and it was caused by a hidden "enter memorized transaction" popup window, which was completely hijacking navigation within QB after entering the password.  Here's what worked for me:

* Closed QB using Windows Task Manager.

* Restarted QB but rather than entering the password for the usual company file, I clicked cancel.

At that point I was able to maximize the QB main window and then open the company file the "old fashioned" way.