Which hedgehogs are in need of help?
Those that need help are:

Injured hedgehogs
Often the place and circumstances (road, construction site) in which the hedgehog is found will alert you to its injuries. Animals which have been trapped possibly for days in trenches, light wells etc without food and water are also in need of help.

Sick hedgehogs
You can recognise sick hedgehogs in that they walk or stagger around foraging or lying out of their nests in daytime. In warm weather swarms of flies lay their eggs on sick, injured and baby hedgehogs. Sick hedgehogs are sluggish and can barely roll up, they are often very thin (they go in behind the head and their hip bones stick out). Their eyes are not prominent and bright but are sunken in and half shut.

Orphaned baby hedgehogs (hoglets)
Baby hedgehogs found out of their nest in the daytime, with their eyes and ears still shut, possibly feeling cool to the touch, have lost their mother and are in urgent need of help.

Exceptions
Exceptions are usually hedgehogs whose nests have been disturbed. Hedgehog nests can be destroyed if piles of leaves or branches are removed, log piles dismantled, work done in parks and gardens, construction work started, or dogs allowed to rummage around. Then even a healthy animal will go looking for a new hiding place during the day. Sometimes during the day, lactating hedgehogs will move between the breeding nest and another separate day nest.