Upstairs Hot - Downstairs Cold ???

Can any body help ? my upstairs rads are hot while the ones downstairs are cold, I've tried bleeding them without any joy, also the pump upstairs is red hot too, the system is a "normal" (dont know what you actually call it) one , not a combi. The whole system was working ok last week,then we turn the heating off as didnt need it on (not the boiler) turned it back on today and thats when my trouble started, I thankyou in advance for any help given