Welcome to Apple Support Communities. We're users here and don't speak for "Apple Inc."
First consult this help file:
If these steps don't work, then yes, it's possible that you've lost "everything", but in many cases, if the hard drive was not actually damaged some erased data can be recovered.
If your data has been erased, there are disk utility programs and there are very expensive data recovery services that might be able to retrieve some or all of your data files. I've not used any utility programs nor any data recovery firms in years, (because I learned my lessons about backups decades ago) so I won't make specific recommendations.
Any reputable data recovery firm will examine your hard drive and give you a free estimate of how much of your information they can recover and how much it might cost.
In any case, I hope you have learned several valuable lessons about how critically important it is:
1. To have current backup copies of your irreplacable files at all times.
2. To have multiple backup copies of EVERYTHING, before attempting any update or upgrade.
3. To read and understand what any specific 'update' or 'upgrade' does to your device, exactly how it works, exactly what benefits it offers, AND, if you still have any questions, TO ASK FIRST in these support forums or elsewhere, and get all your questions answered, BEFORE plunging ahead.
Ancient computing proverb: You can never be too rich or have too many backups.