Vauxhall Corsa.. Head Gasket Blown?

I do not know anything about cars, bear that in mind. Ha-ha.

 I drive a 1997 Vauxhall Corsa (I'm a student) 

 I went to drive to work this morning and realised my
engine temp needle was going up and down up and down,
until it hit 150. So i went to Asda and bought engine coolant,
popped the bonnet up and saw coolant tank was completely empty.
As my engine was a bit hot after the short journey i filled it
half up and drove to work. The temp then went back down to
normal around 90. When i got home I opened the bonnet again
to check i had put enough in but it was completely empty again?
 I checked for leaks and couldn't find any. 

 Please can someone tell me whether it is likely the head gasket
has blown or if any other problems could be causing this. Thanks