Something must have emptied the radiator and then the engine overheated. That may have cooked either the head gasket or the head itself.
Steps:
1) Determine if you want it fixed. Some don't, depends on $. You can get a brand new head for under $1500 with valves and camshafts and everything, just plonk it in and away you go. Assuming you want to fix it:
2) Find where the coolant leaked. Pressure testing is best for this, don't stop searching after the first "found it!!" because the first one might be the head, and the head might be leaking because it overheated. If the coolant leaks out again you'll have a problem again. Do NOT stop at what you can see. Undo the EGR tube (gold tube) from the EGR valve side and continue the pressure test. If coolant is emptying into the intake manifold you'll have even more trouble on your hands later. Same if it's emptying into the exhaust.
3) Assess the damage. The pressure test should point to where this is. If it's the head, you COULD get it welded up and machined and get all the valves redone and ... you're spending a lot of money doing that. Weigh that up against the replacement head. If it's a corroded EGR tube, consider removing it, blanking the ends and connecting the heater hoses together with a short piece of pipe.