If you have found a crow in urgent need of help, then contact the nearest rehabilitator. Unfortunately, there is no general number that you can call.
The RSPCA (England and Wales), SSPCA (Scotland) and USPCA (Northern Ireland) are the national charities that help and advise on injured wildlife. You can also find an independent local rescue centre on Help Wildlife

It is difficult to assess whether a bird needs help. If a bird is a so-called “nestling” (newly hatched individual), it will not survive without help.
As expert help is required, you need to contact a bird rehabilitator immediately. Make sure the bird gets warm with hot water bottles. If you do not have hot water bottles, take soft drink bottles and fill them with hot water. Place a towel on them (so that it does not get too hot for the bird) and place the bird on them in a box. Make sure that no pet can access the bird. 40 degrees is a good temperature.
Those birds cannot control their own temperature, so you have to have a source of heat, else they will die. Just a blanket does not help!
A blanket does not provide any heat, it just keeps warmth in place.

If the nest is still up there, then it may be that something has happened. It could be a predator or a storm that is the reason the baby bird has fallen down from the nest. Sometimes the parents make a decision to throw a bird with problems out of the nest (disease, food shortage).
Sometimes, high temperatures make small baby birds jump down out of panic.

Because crows have their nests high up in the tree, it is usually impossible to reach the nest to check the situation, if you do not have family members in the fire brigade 🙂

Nestling

If the chick is older and a so-called fledgling, a juvenile bird that is about to leave the nest (ready to fly), the parents are usually nearby and take care of it. The problem, of course, is that you do not know.
But if an adult crow starts screaming when you are there, you can hope that it will take care of the baby if you put it back.
Call your nearest local rehabilitator to check the situation in your particular case. Keep in mind that a baby rook does not get fed by the parents

When it comes to corvids, Rooks are an exception. They do NOT take care of their young on the ground that can not fly.
Important… If the rook can fly a little, then all is fine! Just let it be. It will be ok even though it does not appear that way!
If the rook baby is alert, you can try to put it back on a branch in the tree that is a few meters up (via a ladder or balcony nearby?). If this is not possible, you can also try putting the bird on a broom or branch and lifting the branch as high as you can reach (the higher the better). If you have succeeded, you should leave immediately and stay, preferably at a long distance.

If you see an adult rook visiting the young rook, you can say that you have probably succeeded in rescuing a baby rook. It can take an hour before everything has calmed down, so do not immediately think that you have failed if nothing happens.
The rook parents do not like you, so it may also be that they wait until you are gone. In that case, try to go further away and check from a distance with binoculars (preferably indoors if possible).

Young rook (The feathers on the beak show this is a young rook, they loose the feathers on the beak after around 1,5 years)