Well it's cooled down now and dark, sorry for the delay.
When cool is best, to avoid thermal shock & potential component cracking.
When was the coolant last changed.I would be tempted to fully drain, flush and refil if you lost a lot. Otherwise, top up with a good strong mix, say 40-50% antifreeze.
Make sure you don't mix coolant types. Blue with blue, pink with pink.
Fill to the max mark, leave the cap off, heater on full hot, let the engine get warm and the coolant start to flow through the header tank.Top up as required and then replace the header tank cap. Hopefully any air lock will be gone by then.