Most likely you signed into the wrong shard for your account.
Overtime, as SmartThings has added many more customers, they have added additional “shards.” ( you can think of these as database servers.)
You wouldn’t think that SmartThings would let you sign into one that’s not associated with your account, but it does, and if you do then you won’t see any of your devices or smart apps.
There are still many posts in the forums back from the time when there was only one shard and so the direct link may not work for you.
See the following FAQ to figure out which link you should be using and then try that.