Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if a sudden smell change occurs with:
Facial drooping, arm or leg weakness, or difficulty speaking
Severe confusion, collapse, or a sudden severe headache
Significant head trauma, repeated vomiting, or increasing drowsiness
Breathing difficulty or illness after smoke or chemical exposure
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Arrange same-day medical advice for new smell loss after a head injury, even if the injury initially seemed minor.
You should also seek prompt care for clear fluid draining from the nose after an injury.
And for anyone, someone who:
has a wound that is hot, spreading red, or leaking pus
3
Does any of this apply?
Book an appointment
Book a routine appointment if the change persists after a cold or nasal congestion has cleared, lasts for several weeks, keeps returning, or affects eating and daily safety.
An appointment is also appropriate if you have one-sided nasal blockage, repeated nosebleeds, facial swelling, or worsening pain.
The article also says
A brief change during a straightforward cold can usually be monitored at home.
Follow current local advice about respiratory infection testing and reducing the spread of infection when relevant.
Every sign above is quoted from the sofidex article on Smell - impaired. If you are not sure, err upward — it is always reasonable to be checked.