The servers for PS3 third-person shooter Starhawk are now closed, having been taken offline at the end of June. Here is the official line from the PlayStation Store:
"From 30 June 2018, Starhawk’s multiplayer features will be terminated and no longer available; but its single player campaign and any other offline content will continue to be playable."
Unfortunately Starhawk had a significant online component, and so 18 of the game's trophies are now unobtainable. Only players who have already completed those trophies will be able to get their shiny Platinum.
You can see a full list of the discontinued trophies here.