Why is my phone locked after I took out SIM card and put back in my Samsung?

[Android Central Question]

I have a Samsung Galaxy s5 I bought at AT&T a few years ago. I switched carriers and was using another phone until this week. I was told I could use my Galaxy s5 and just switch SIM cards. I tried but they were different sizes. So I put the original sim cards back where they were originally. After I did that, my Galaxy S5 now wants me to put in a numerical password. Before, I took the sim card out I had a dot pin. So why did it change just by taking out the sim card and putting it back in? I don't have a number pin and never did. I tried like 20 numbers (not all at once) and it seems impossible. I went to ATT and they cant help me. I called Samsung and they told my to go to findmyphone.samsung.com and unlock that way. I did all that but it couldn't find my phone because I guess I didn't ever register it. So I don't want to reset the phone at a Samsung store because I have over 500 songs I downloaded and pics and videos I don't want deleted. Can anyone tell my why this happened and what pin number I am suppose to enter?

 

[Almeuit]

No one can really tell you what pin to enter -- we wouldn't know.

 

[Rukbat]

Or even why taking the SIM out, then putting it back changed the lockscreen (which is why AT&T and Samsung can't help you). Your only option at this point is a factory reset, and learning that any file you don't have backed up is a file you don't need. (And I'll bet that line is older than you are.)