Hello and welcome to Just Answer
1st question to ask is where is the coolant gone?
as if it has failed a head gasket then it may have been forced out the filler cap vent
this would also prevent it from starting
So I'd begin with a compression test across all cylinders to ensure that they are all within 10-15 psi of each other
if this is Ok then
If you have a tiny pinhole leak in a radiator then these often don't leak water but will lose water as steam ( steam molecules being small enough to pass through the hole where liquid cannot) take a look on the rad for any signs of localised corrosion and for areas that look like they've been 'steam cleaned'.
If all of this is OK it may be worth replacing the header tank cap as again you can lose coolant as steam if this is blowing off too early ( the cap should hold pressure of around 1Bar)
The other thing to try is a head gasket / block test kit as these take a coolant sample and analyse it for any signs of combustion gas - if present this would indicate the head gasket is on its way out
and as a fix for internal leaks I’ve found this product works very well, without clogging heater matrixe’s
it doesn’t last forever though