Has it fallen over or been knocked over?
If a milestone has fallen over or been knocked over and is intact, it needs standing upright again. Milestones are heavy objects and it may require several people or lifting equipment to set the milestone upright. Here again, health and safety become a priority and careful planning is needed before work is undertaken, including checking for underground utilities services.
The milestone will not stay upright just balanced on the surface of the verge. Typically, between a quarter and a third of the milestone will be underground to give it stability. There is often a clear line ‘ground level’ on the stone, leaving an undressed part to be buried.
It is not recommended to set milestones in concrete, because, although this will prevent them being knocked over, if/when they are hit by a vehicle they will be broken in two or possibly destroyed. You could put a concrete ‘collar’ around the milestone, perhaps with inset slabs or crazy paving, to stabilise the ground, prevent weeds, improve visibility to verge cutters etc.