1996 Ford Mondeo Head Gasket?

I've got a 1996 ghia x 16v zetec. About 2 days ago I noticed the coolant level was low, so I topped it up (took 1 litre). Then on the motorway today white smoke started pouring out of the exhaust, so I imeadiately pulled over and it cut out before It came to a standstill and then would not start again. I then checked the coolant level and it was completely empty? Any suggestions on possible faults, head gasket?

thanks in advance

Jake