If there's no sprint reports at all - are you seeing only burn-up, burn-down, velocity chart and a cumulative flow diagram?
If yes, you're likely in a Next-Gen project as opposed to a Classic one. Whilst it is possible to re-open sprints in Classic, this is not yet a feature supported in Next-Gen. You can vote for and follow the progress of this request via JSWCLOUD-17208.
If you just need to view the issues which were in the sprint, you can do so via the "Sprint burndown chart".
Let us know if this resolves your query - if not and you're in a Classic project, please send us a screenshot of what you can see under Reports.
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