UTorrent contains a "boss key" feature. When pressed, the boss key combination hides the uTorrent window and its system tray icon. UTorrent runs in the background; the only indication that it's running is the uTorrent.exe process in the task manager. UTorrent won't respond if you try to reopen it while it's hidden.
Press the boss key combination you set in uTorrent's preferences window to show it after it's been hidden. UTorrent doesn't come with a default boss key setting, so you'll need the key combination you set. Depending on your uTorrent settings, you may also be prompted for a password. You can close uTorrent from the task manager if you don't know its boss key combination.
Forcing uTorrent Open
Open the task manager by right-clicking the task bar and clicking "Task Manager." Click the "Processes" tab in the task manager window, click the "uTorrent.exe" process in the list, click the "End Process" button and click "End Process" again to confirm. Launch uTorrent again by clicking its shortcut on your task bar, Start menu or desktop. You can configure uTorrent's boss key settings after reopening it.
Boss Key Configuration
Configure the boss key settings from uTorrent's preferences window. Click the "Options" menu in uTorrent, select "Preferences" and click the "General" category in the preferences window that appears. Click the "Boss Key" box and press a combination of keys to set your boss key combination; you can also press "Backspace" or "Delete" to disable the boss key feature. Use the "Use Boss-Key Password" field if you want to set a password. You'll need this password to unhide uTorrent.
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Boss keys are special keyboard shorcuts that hide a program when pressed. While the name suggests hiding the application from your boss at work, you can use the boss key to hide uTorrent for any reason. Use the boss key combination when you're using uTorrent on a shared computer and don't want anyone else closing it.
Bugs could also cause uTorrent or other applications to persist in the task manager. The uTorrent.exe process may appear to be running in the background, when it's crashed or hasn't closed properly. You can end uTorrent's process from the task manager and reopen it in the same way to fix the problem.