Multiple Dead Outlets, Breaker not tripped
As you suspected your Christmas lights apparently tripped the GFCI. The trick is locating the controlling GFCI. In some homes built in the 70s and 80s garage, exterior and bathrooms are on the same GFCI circuit. The GFCI might be on the breaker as well.
&quot;I also checked the line and load sides of the GFCI plugs inside the house and both appear to be functional.&quot;
It sounds like you've tried all of the GFCI outlets without luck but you probably missed one. If none of the breakers are GFCI protected you need to determine which of your GFCI outlets is first on the circuit after the breaker on the problem circuit. With a multimeter check the line side for power. If there is power on the line side and not on the load side and it won't reset, you need to replace that GFCI outlet. If there is no power coming into the controlling GFCI the problem is most likely at the breaker. It's unusual for a breaker to fail but it happens.