I have built things in the nether and ever since the 1.17 update it reseted! Everything i built was gone! EVEN THE PORTALS ITSELF WERE GONE!

I have built things in the nether (the world was created for 1.16) and ever since the 1.17 update it reseted! Everything i built was gone! EVEN THE PORTALS ITSELF WERE GONE! Can u plz help. It took for ever to build to the presepctive portals.