Mobile phone problem help please

[abitoflight]

Daughters phone
She's away from home with no service at all
Changed provider on Wednesday evening
Phone worked on that provider for an evening
Thursday morning onwards no service
I've checked all I can - sim code, coverage, provider settings etc
I'm asking if her old provider could have locked phone or if it could be locked at all as it worked with new provider for a night?

They can relock themselves but not if it's done through the networks server. If she has contacted you she will know of its locked itself as it will display a black screen asking for a code to be entered.

Thank you
No such screen - just says no service
Also worked on new provider for a few hours - giffgaff
I'm in the process of doing a sim swap for her as she can't do anything with no data ad slow WiFi and I'll drive the new sim down tomorrow