I created the document which is my journal. I did not knowingly put it in 'read only' mode and now I cannot edit

I keep a daily journal on my pc. On a recent trip, I continued my journal on my laptop and then pasted it into my pc. Now I find that I cannot edit and I don’t know how this happened. I would appreciate some help with this but I ask that you make your explanation as simple as possible.

What message do you get to suggest that you can’t edit the file? And how did you actually transfer the file from one machine to the other?