There are 2 support numbers, one is for anything Samsung and the other is SmartThings specific. You will most likely have better luck with the second one:
1-800-726-7864 is Samsung’s main support number in the US.
1-855-238-7339 is a number specific to SmartThings and ADT.
If you can detail your issue maybe others on the forum can help you otherwise this is an unhelpful complaint that nobody can help you with
The most common issue with the migration is simply that a new location gets created without user awareness and the mobile app is defaulted to that location so it appears like everything has vanished. Simply selecting the other “Home” location will fix the issue. I noticed the issue as my default location was not called “Home” so the new name tipped me off… plus I read a lot of posts here.
Another issue is that you may have been moved to another shard. I was on the original one when on v2 and with v3 I am now on a server on the East cost. The URL to login that I was using still takes me to the old shard so once again nothing shows up… try this one instead:
Or… once you are logged in, got to Locations and select the new location to be taken to the right place.
Just a shot in the dark since we have no info on what your issue is… please share it, and you might get a solution from a forum user.