I switched my simti an old phone and now it says phone is blocked on the acct

I switched my simti an old phone and now it says phone is blocked on the acct,,,,

So what old phone?  Is it possible that old phone was involved with a insurance claim and never return to the insurance company? It would then have been blacklisted.  

phone n worked perfectly fine prior to getting repaired.

Who did the repair?

Why is this happening.. also put sim in another phone and asked me for network 16 key password .

If the phone is asking for a 16 digit password, either the phone is carrier locked to a different service provider, or it's asking you for the network key which is a password for your Wi-Fi.  

You haven't really included enough information for us to understand exactly what's going on and why you're having a problem. But if you're sticking your AT&T SIM card into phones that are not AT&T phones, that may be blacklisted or locked to a different service provider, please stop doing that.  

As for the phone that was repaired, who repaired it? If they used parts from a different phone, they may have messed your phone up