Water Tank in loft no longer fills up
It seems your fill valve is stuck in the off position.

I don't have that particular type of valve but I'd try wiggling the moving parts (the blue parts probably) to see if that frees things up.

The instructions show how to check the valve and replace the seals. 



I'd turn main stopcock off, pull that valve cover off, clean all moving parts up and replace them. 

If still not working , you may have to turn off the water and replace the seals with new parts or replace the whole valve with a new unit.