I have a UE40F6320AKXXU with the little joystick rather than an ON/OFF switch. Trying the joystick and the remote control did not work. The suggestion from Crookie to access the Service Mode worked a treat. At last back to having a 'smart' TV. Thanks for that.
Give a chance to the REAL hard reset:
Turn OFF your TV. Press the physical ON/OFF button on the TV (most probably the joystick button on the back) for at least 3 seconds, then press the ON/OFF for at least 3 seconds on the remote within 10 seconds. TV must restart 2 times, and goes back to virgin factory mode. By other words, it will erase the EEPROM (store of the apps), settings.
If you success, you should have again at least 30-40% free memory which is enough to download (and store) the HUB app.
Yeah, and switch OFF the auto update of the installed apps...