Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care after a facial or head injury if you have:
Seek same-day medical care after a nasal injury if you develop increasing blockage or a soft, tender swelling inside either nostril.
These can be signs of a septal haematoma.
Increasing redness, warmth, pus, fever, or worsening pain also needs prompt assessment.
Book a routine appointment if the shape is new or gradually changing, or if you have ongoing nasal blockage, repeated nosebleeds, crusting, pain, or concerns about appearance.
A child should also be assessed if the feature occurs with feeding, hearing, growth, or development concerns.
The article also says
A stable low bridge that has always been present and causes no problems does not usually need medical care.
A new bridge collapse without an injury is not usually an emergency, but it should be assessed promptly, particularly if it occurs with persistent sinus symptoms, joint pain, skin changes, fever, or unexplained illness.
Every sign above is quoted from the sofidex article on Low nasal bridge. If you are not sure, err upward — it is always reasonable to be checked.