Laguna MK1 engine stop and temp light

Hi all.

My Laguna MK1 1997 "Engine stop and temperature" lights have just turned on and I can't think to reason why.

I be just replaced the head gasket, a few days ago and all was fine until this evening.

I've checked the water levels and they seem to find the problem. I did change the coolant this morning.

The temperature gauge dropped in temperature below the half way mark, but no signs of overheating.

Thanks for anyhelp

Laguna MK1 engine stop and temp light