Hydrangeas planted on the previous year (mopheads, oakleafs, etc) have been known to leaf out late on their first Spring. Wait until the end of May to decide whether the shrub made it or not. Dead stems should trigger new growth from the root system (if the roots survived) so, it is good to know that the stems showed green when you scratched them. To help, keep it mulched (2-4" ) and keep the soil as evenly moist -not wet- as you can. Water deeply from the root ball outwards so the soil feels moist to an approximate depth of 8" .