If you suffered through reading the other posts on injuries then it is time to bring it together with discussion of how to return to activity after an injury has occurred.
First and foremost - Re-read the the other posts - Recognize that an injury has occurred and do what you need to to deal with it. This section of this line of thought assumes that you stopped as soon as you recognized a problem or injury occurred. You applied the appropriate first treatment steps (Ice, Rest, etc...) and that you followed up with your health care professional (Dr., Chiro, PT, etc...). And most importantly assumes that you followed the advice/directions given by said health care professional.
So returning to activity assumes all of these things and that you are "finished" with any pain and/or rehab. Don't become one of those people complaining of a "bad" shoulder that they injured 5 years ago and just thought it would get better and don't fall victim to the recurring injury or the "it's just going to hurt" mindset - all of these are the path to joint replacements and loss of function.
So you are pain free and have clearance from your health care professional - What now?
Do NOT jump back in to your previous program!! If you sustained a non-contact injury then you have an underlying restriction or asymmetry to address. Hopefully you were screened for this and had this addressed during your rehab/recovery time. If not you will need to get screened and address the restriction or asymmetry.
After that - What to do?
If it was a lower body injury - you should be able to deep squat, toe touch, single leg stance and single leg hop for reps as a minimum to returning to activity.
If it was an upper body injury - you should have full overhead movement of the arm, be able to perform planks (on elbows and straight arms), be able to perform a push-up, renegade row, and finally a clapping push-up.
If any of these produce pain or symptoms - you are not ready to go back to your previous routine and you need to re-evaluate your injury and begin again.
Once you can pass the above recommendations:
Start slowly and lowly - Reduce weight, sets, reps and everything to a nice low starting level. After my back surgery my recovery looked something like this - First 4 weeks - walking, Second 4 weeks - bodyweight exercises (push-ups, pull-ups etc...), Third 4 weeks - light Kettlebell swings and other KB drills, followed by a nice gradual progression back to my previous workloads.
Any of the above information is not intended to replace any medical advice or treatment - Always consult your physician regarding an exercise program or changes to your exercise program.