Try below in order and move on to the next step if your issue is not solved:

1) Right-click the task bar and click "Cascade Windows". Check if you can see the wallet.

2) On Windows:

• Exit wallet
• Run "regedit" (Press "Windows" and "R" keys together, it will open a run window, write "regedit" there and click "OK")
• Delete this reg key:
  HKEY_CURRENT_USER\Software\ColossusXT\ColossusXT-Qt\nRPCConsoleWindowPos
• Restart the wallet and the window position should be fixed.

On Mac:

• Exit wallet
• Open Terminal
• cd ~\Library\Preferences
• rm io.http\:\ \ colossusXT.ColossusXT-Qt.plist
• Restart the wallet and the window position should be fixed.