Cellphone Damage/ blocked by store employee

My phone was working absolutely perfect. I needed a Sim card to be able to use an apple watch I bought from them. So the only thing they had to do from what I know was to change a SIM card from one to the New one. Well when they gave me back my phone they told me the phone was blocked and they couldn't do nothing about that... when I turned on the phone the only thing that said in the scree was "Hello".. So I asked them what happened and they couldn't give a straight answer.... since I don't know much of technology ... I ask for them to give me a written report of the error  they told me they weren't authorize to do that... so I ask for the manager or they superior which never came to talk with me... so again I ask to someone I can called about this.... they told me that it was not protocol to give me that kind of information.. they gave me 3 options.. #1: Buy a very old model of iphone for $5...#2: but Iphone 11 or #3: try to backup my phone to see if it works...I choose #3. In conclusion the phone never worked... I returned the watch.. So overall i lost all my day in the store and I left without phone and watch for something I have absolutely nothing to do with. They never took responsibility not acknowledge there error not even try to explain what happened. I was astonished... it very hard for me to believe that AT&T doesn't have protocols in situations like this. I really don't know how to proceed.... something I can do?

My phone was working absolutely perfect. I needed a Sim card to be able to use an apple watch I bought from them. So the only thing they had to do from what I know was to change a SIM card from one to the New one. Well when they gave me back my phone they told me the phone was blocked and they couldn't do nothing about that... when I turned on the phone the only thing that said in the scree was "Hello".. So I asked them what happened and they couldn't give a straight answer.... since I don't know much of technology ... I ask for them to give me a written report of the error  they told me they weren't authorize to do that... so I ask for the manager or they superior which never came to talk with me... so again I ask to someone I can called about this.... they told me that it was not protocol to give me that kind of information.. they gave me 3 options.. #1: Buy a very old model of iphone for $5...#2: but Iphone 11 or #3: try to backup my phone to see if it works...I choose #3. In conclusion the phone never worked... I returned the watch.. So overall i lost all my day in the store and I left without phone and watch for something I have absolutely nothing to do with. They never took responsibility not acknowledge there error not even try to explain what happened. I was astonished... it very hard for me to believe that AT&T doesn't have protocols in situations like this. I really don't know how to proceed.... something I can do?

When you see the “Hello” screen, it usually means that the phone has been factory reset and you just need to go through the setup again.  Did you go through the phone setup again?