diving along and apparently ran out of coolant. temp was 260. now it climbs to 260 real fast. what is most likely problem??

The most likely problem is the water pump. Is there any coolant leaking out? If it got that hot you may have warped a head as well.

Coolant had to go somewhere.

Refill the system and bleed out the air through the bleeder at the thermostat.
When overheated the thermostat will almost always fail and remain closed. Replace the thermostat.
If still getting hot:
Pressure test the system and see what shows up. If nothing visible, warm it up with the rad cap off. Look for coolant flow to assure that the water pump is indeed moving the coolant. If you see flow and surge in the fluid when reving the engine, then the water pump is ok.

Next look for loss of coolant into the engine... ie through the cylinder heads ie gaskets warped head etc. Usually see some white smoke out the exhaust if you are burning coolant. Then consider pressure test the cooling system with the spark plugs removed. Wait a while and see if the pressure drops... crank the engine with the coil disconnected and see if coolant spews out of one or more spark plug holes = head gasket or warped head.

let us know what you find.