Baseboard water heat not heating one room; Rest of zone/house is good
First, if this is a forced hot-water system with baseboard heating, they're called convectors, not radiators.

Bleeding the system is fairly simple (though a bit messy and time-consuming), and, as you diagnosed, the most likely fix. There should be an air bleeder, or purging, valve at the end of each baseboard unit and one in the basement by the boiler, perhaps near or on an expansion tank.



Some valves are automatic, having an insert that expands when wet, effectively sealing the valve, and contracting when dry so air escapes. If that is the case, you can wait, and eventually, most air would bleed out... but that would take quitre a while, if your system had lost most of its water.

Manual and automatic bleed valves can also be loosened by hand, with a screwdriver or coin, or with pliers. Use caution, because an old fitting could be brittle and break if forced. Lift up the baseboard cover from underneath and swing the bottom out to access the valve at the end of the pipe. Have some rags handy, as there is inevitable water leakage while bleeding. Also, mind the hot water.

Though it's usually easier to access the main bleeder, it's at a lower point in the system and will not get out the last bit of air blocking heat to that room. However, it may be useful to purge some air from that, too.