I started yoga again after a long break. What stretch is safe for my body?

Great!  Yoga is good for improving flexibility and core strength.   Unfortunately I am not an expert at yoga.  I recommend discussing your workout routine with your yoga instructor.  As in with any exercise it is safest to start slow and slowly increase over time.  You should try to increase your stretch over many days or weeks as you recondition your body.  You want to avoid very painful stretches to decrease the chance of tearing soft tissues and injuring yourself.   Good luck!