Probably you accidentally moved it offscreen (or changed display settings such that the original location no longer exists). See the Handbook under "Revert to factory settings" to restore the default position. if this does not solve the problem, pleae ask for further assistance in the Support forum (eg, Help / Ask for Help from within MuseScore) and provide more information such as your OS.
Comments
Probably you accidentally moved it offscreen (or changed display settings such that the original location no longer exists). See the Handbook under "Revert to factory settings" to restore the default position. if this does not solve the problem, pleae ask for further assistance in the Support forum (eg, Help / Ask for Help from within MuseScore) and provide more information such as your OS.