Is your downstairs box working okay?
Check the coax cable is screwed in tight both at your box and at the wall point. Switch box off for 10 minutes, reboot and wait 15 minutes before trying to access on demand. If it's still not working it may well be a box fault. As this is a V, V HD or V+ HD which is being phased out, troubleshooting is limited and Virgin will look to replace it with a TiVo box in the first instance.
Ring 150 from your landline or 0345 454 1111 from other phones, use the option for "if there's a fault with your TV service" and see what they say. If they can see signal issues present they'll send an engineer, if not they'll post you a new box.
Let us know how you get on.