Yes, you probably have to train again.
The kernels don't know about notebooks. They send their output on channels, where it is received by the browser, and stored into the notebook you are working with. If the browser doesn't receive the output, it will not be stored.
If your kernel is still running, then the results of your training may still be in the memory of the kernel. Then you could try to add another code cell which prints the results of the previous training. But this will only work if the same kernel instance is still running, not if you're starting a new kernel instance from the same notebook.
hope that helps,
Roland