If there are any leaks in the system (an air leak in is as bad as water out), there is probably air blocking the water from circulating to the upper floor's radiators. Is there a bleed valve on any of the radiators? You need to open that until water starts to come out. That is, if the system has an auto fill valve to keep the system properly filled up with water. If it doesn't, you need to open the manual fill valve. It can get tricky, since the pressure in the system is normally only about 13-18 pounds or so, and your potable water system could be three to four times that. You don't want the pressure to get too high.
It can get a little tricky, and if you don't do it in the right sequence, you can trip a safety interlock or never get it to work. If you don't know what's going on, it can get very frustrating and cause the whole system to shut down until you bring everything back to within tolerance. Things that can trip the safety are: too much pressure, too little pressure, too high a temperature, and many have a max duty cycle where they'll just turn off for a few minutes if they've run longer than X. If the sytem has a flame detector, that's also in the safety circuit - it won't leave the gas on if it senses there's no flame.
If bleeding the radiators upstairs doesn't fix it, the best thing would probably pay someone to check it out for you, and have them explain and show you what's going on. once you get the hang of it, it's not that big a deal, but there's a lot that can go wrong if you don't understand, and it could put the system into a situation where it won't run at all - not a good thing as winter approaches.