i can't to startup my imac.

i can't to startup my imac. so, i try to recovery, mounting.................(and other) on the macintosh hd in disk utility but all fail. this problem cause me cannot reinstall os x and also can't to startup it. i have no idea with this problem, i try all the way that i know but failed. so my last way is get help from other people. i try to online chat with the apple support, but i lost my serial number, so can't. then, i try to contact them by using phone but using phone was not clearly to hear what they talking about. so, anybody can help me to ask the apple support online??? please..... 😟