Young, on ground
If lively and uninjured, it may have fallen from a nest or branch.
Check nearby trees for signs of occupancy; streaky white droppings, pellets, etc.

Lift the bird up to as high a branch as you can reach and observe it from a distance.
Little Owl - these sometimes nest near the ground in tree stumps so it may have wandered away from home.
If it is very wobbly and unwell, keep it warm and quiet and take it to a rescue centre as soon as possible. 
If it lively and undamaged, check on it near dusk, when the parents should arrive to feed it. 
If the parents do not visit it with food, it must go to a rescue centre as soon as possible for feeding.

Standing/lying at roadside
It may have flown into a car or been knocked down by the side wind from a passing lorry.
Pick it up carefully and avoid the talons. 
Wrapping a small towel around the lower body is the easiest and safest way, then the bird and towel can be placed in a box or cat carrier. 
Keep dark and quiet and take to a rescue centre as soon as possible as it will need fluids, treatment for shock and raw food with fur or feathers.