Hi Nicola. From what you have said, it sounds like the back brakes (the only ones used when the handbrake is applied) have become stuck 'on' at some point. You will need to get them checked A.S.A.P.You may need just new rear brake pads or possibly new brake drums/discs if badly worn. Some of the big 'tyre' companies will do a free brake check.
Don't worry but do it soon as braking is seriously effected with only front brakes working.
andywrekin
October 2008