The pressure relief cap on the header tank is often the first thing to go wrong, there are both OEM & aftermarket versions available, most of the aftermarket ones are rubbish!
Once you've lost coolant you'll need to bleed the trapped air out of the cooling system.
There are 3 bleeding points, you should be able to find a "how to" guide on this forum.
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"