As said , first find out if there is water going to the heater at all . Then open all the heater controls to full , unclamp header tank and hang as high as poss with string from open bonnet and fill up . Run engine , see if any air bubbles back out of tank . Fit a new stat cos they is cheap . Bleed air out of bungs replacing with brass central heating ones while there . Come back when you done all that and tell us what happened .