Don't feel bad, no one would have known without proof. They've changed their profile name is all. If you can't access their account, they've most likely blocked you, as I've realised that's what they've done to me, lol.
I'm not out to demonise or (what seems to be the norm nowadays) 'cancel' them. All they need to do is show real remorse for their actions and apologise for their mistakes, then they can move forward from this without hiding.
I've seen something similar happen on Tumblr where someone traced a pretty popular artist's work, but it was recognised pretty soon in that community. The person apologised to the original artist and owned up to their mistakes, explained that they admired the artist's work and were trying to get better at their own art. Now they were going to put in the work by actually studying and practising (I will note that it's okay to trace and reference from others for practise on your own time, just don't post it and claim it as your own, but if you do, credit the original artist). The artist admired them for that and told them all she wanted to see is them putting in the effort and improving. She encouraged them to tag her to share their progress (I believe she'd share her work on her blog at times too? I wasn't too involved when that incident happened) and everyone learnt and grew from that. In some areas, the art community can be pretty harsh and damning, especially when things like this happen, but as a whole, it is supportive and encouraging. Everyone wants others to grow and succeed in their own way, even from a blunder like this.