Usually once a period of paddock rest (advised by your vet) combined with physiotherapy will see a horse with sacro issues come back to riding although this greatly depends on the cause and severity of the issue... some may be fine for hacking only, some may be lucky enough to return to jumping however the big stuff is not advised. You need to slowly rebuild strength in the area . If you are on Pinterest you can fund a lot of great articles and exercises for strength building the sacro and surrounding muscles.
My advice, don't be in a hurry, take it slow and follow the advice of your vet and physio .
Good Luck with it
I would ask your vet about getting a physiotherapist. They can help release the compensatory muscles, help mobilise the joint and develop the pelvic muscles. They can also give you stretches.
Do you know the cause?