I wouldn't panic about the opening. Not yet. It is not unusual for the opening to written or rewritten after everything else is done.
For a start the exact feel of the story is not always clear until the book is almost done. In my own first book I wrote a prologue well after completing the first draft (even though the scene had existed in my head from very early on), and I didn't actually decide to use that prologue for even longer. In my second book I pulled a scene from my first chapter and used that as a prologue, after I'd finished writing the story, because only then was it obvious that the the scene didn't belong where it was.
Whether all or any of these choices prove to be good ones remains to be seen, but the point is that where and how you choose to start a book remains flexible. Somewhere I read the comment that there was nothing wrong with most first novels that dropping the first three or four chapters couldn't fix. I can see at least an element of truth in this, even in what I decided to publish, but - for better or worse - I still chose to tell the story the way I wanted it told, and to hell with the theory. Which leads me to...
Like most writing rules, the advice about the opening of the book needs to be tempered with a few practicalities. If you're writing an action thriller or horror story then getting right into it is, I guess, bordering on critical. It's what your readers have come to expect in this this world where so many such books feel effectively the same. But with other genre's there is still some room to move.
Obviously you must still interest or intrigue your reader very quickly, but that doesn't always involve looking down the barrel of a gun from the first sentence*. Always remember that it's your book. Don't bore the reader, especially not in the opening, but it doesn't always have to be a battle from the off. Suit the opening to the story. Intrigue the reader, but set up your story as you want it to feel.
* I think it was an Alistair Maclean novel where I first remember that as the opening scene, but I can't remember which one, or what happened next. Dramatic opening scenes may grab the reader, but they don't necessarily make for a memorable book.