Remote access from one Mac to another; cannot control remote computer

I use my Mac Airbook to remotely access my dad's (85 years old and needs help sometimes) Mac desktop.  Up until today, everyone has worked fine, but today, it would not let me control his computer.  I could move my cursor (I use a touchpad) but couldn't click anything.  We are both on High Sierra, and are using the most updated Team Viewer.  Any help would be appreciated.