what YOU can do
Give them their space; if they are older allow them to decompress in a safe place, if they are younger give them enough space but maintain supervision.
If they have run away and are missing, check the places they may potentially be and/or call the people they may be with.
Let them know you are there for them - that your priority is to keep them safe.
If you know their location, pick them up. It is the parent's responsibility unless the youth is resisting, at which point it is appropriate to call 9-1-1for help.