The radiator will still work regardless of the direction of flow. The arrow is only there to stop the valve hammering when water flows through, but this was really only a problem with older valves, most new valves have a double headed arrow.
If you have drops i.e. the pipes to the radiator drop down from the ceiling rather than rise up from the floor then you most likely have an air lock in the bend just above the ceiling (assuming you've checked everything in masona's post). Best way is to turn off all other rads, find the pump and turn it's speed up to max, usually 3, this should push the air through. If you have a timer that allows a choice of ch, ch/hw or hw on its own, then make sure only the heating is turned on to maximise the flow through this one rad. You may also notice a bypass pipe, it should be directly below the pump (the pump will have an arrow on its body). This small bypass may have an isolating valve on it, so make sure this is also turned off but remember how far open it is.
Many of these instructions may not apply to your system as you have not indicated what sort of boiler you have.
Regards, Paul
