Upstairs heating working - downstairs not. Very cold, help!

[egi]

Hi,
we have a prob with our gas central heating. I turned one radiator downstairs off the other day (not sure if that is what cuased the prob). A while after turning it back on, the valve or pump (sorry can't be sure which) starting making a straining noise as though it were trying to turn over but was struggling. The noise eventually stopped but all the downstairs radiators stopped working!
We checked for an airlock in the pump by letting air out of the bleed valve.
We then turned off the upstairs radiators. The downstairs ones then came on (well, one did fully, the other was just warm).
However when we then turned the upstairs ones on too, once again the downstairs radiators went off.
Sorry for this rather long winded and not very tecnical description!
Does anyone have any clue as to what we should try next please?
Thanks

 

[gas_man_gaz]

check ya lil tank in the loft and make sure theres water in it, but it sounds 2me like ya pump has gone or ya hav a blocked cold feed or both

 

[egi]

Thanks will do that