Did you try filling it LGF??
Might very well just be a drop in pressure,as its been off and unused for a while
1,Look under the boiler
2,You will see a small hose connecting 2 pipes[there should be 1 or 2 taps connected to them]this is the filling loop
3,Turn them till you hear rushing water[thats it filling]
You may have to open one tap then slowly open the other till you hear the water[experiment as to which tap you open and which you open slowly[till you hear..etc.]
4,when you hear it filling,leave it for a few seconds [It Takes only about 5 seconds so dont over do it thinking its not enough]
*Take note*
When you call the plumber in,this is the very first thing he is going to check
IMHO,i dont think its the thermostat,if that was the problem you would get nothing,it wouldnt come on at all