No hot air, coolent related

[Brad.]

So other morning I got In the car to go to work and needed to demist it, but was only cold air!

Had some rummaging on the web and seemed to point to coolent, or lack of. So I topped up my water to the maximum mark.

Had heaters after that! Until this morning, same again! So got home just now, checked the coolent, looked empty, cracked the lid and it allll came rushing back in and is full again! So I know I've got an airlock/blockage somewhere.

Common areas? Checked both pipes on rad and both equally as hot, engine ran fine temp wise. Hoping its just a stat swap but open to opinions from all.

Thanks.