valve closed & no heat in zone one

After a particularly cold night, we realized the heat was not working in two of our upstairs rooms, call it zone one. The other two zones were heating fine. In the morning, I went to the basement and realized the t-valve on one of the pipes coming out of the furnace was in the off position (perpendicular instead of parallel to the pipe). We had several plumbers doing work in our house the week before and assume someone turned it off for some reason and forgot to turn it back on. The pipe in question is hot on the side leading out of the furnace and get progressively cooler past the "off-valve" as it routes around the basement ceiling and up through the floor joists - I assume on its journey to the upper floor where the heat isn't working! However, it would have been off for several days at this point, and I know nothing of the consequences of just turning the valve back on. I assume if I do, the heat will work again though! Do I need to worry about anything or is it safe to turn it back on? Is there built up pressure in the pipes? Will air get in the pipes? I don't know that I can bleed them, because I can't find any valves on the floor radiators upstairs. I looked at all exposed ends and they all just go straight into the floor! Zone one has baseboard floor radiators, not the old school ones that are in the other two zones in my house. Please help, as I don't want to spend any more money on plumbers!