2004 Vauxhall Corsa heater not working and car overheating

First thing to check would be water / coolant level and top up if needed.Could be something simple like jammed thermostat or major like head gasket .Got to get it checked out before you ruin engine . If losing water , a pressure test will show any external leaks and a sniff test ( block test ) will tell if head gasket gone .Hope this helps .

Check if there's a return on the coolant filler tank, had this problem on a Honda once and the return was blocked as it went into the filler tank, unblocked it and it worked fine.