Many swimmers hold their breath as a natural reaction from having their face in the water. Others were told to deliberately hold onto their breath when they learnt to swim - and then exhale it at the last moment as they rotate to breathe. In fact around 90% of swimmers we meet on our clinics could improve their swimming by developing better exhalation technique!
Once you have sunk down, as soon as you feel low on air push up towards the surface again. Take 10 or 20 seconds to compose yourself and then repeat a few more times.