This happened to me on my first solo
As I reported downwind, I heard another aircraft report long final. Some way downwind I saw him, further out and high. I thought he'd also see me. Getting closer to turning base he was still well above me, doing a fair rate of knots and appeared not to see me.
Extending downwind is discouraged at Elstree, so I went around. He never made any signs of having seen me. Tower (Elstree is AFIS) had never heard of him before he called 'long final'.
One for the books, but, ultimately, one to learn from. Whether inadvertently or not, these things happen and you may as well expect it. The good news was that one of us at least had seen the other one. Doing an extra circuit is, at the end of the day, a nuisance but not the end of the world.
B.