It is not possible to reopen the Episode Manager after closing it. The graphical window is triggered only when you run a function like train. The windows is destroyed once you close it. If you want to access specific training variables like EpisodeReward, TrainingSteps, you can get them in the workspace as output arguments.
To add to this answer, starting in R2020b, there is a method you can use to recreate the Episode Manager graph using the output arguments mentioned above
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