Github has very simple rights/privileges (and even simpler management for them).
If you are not a collaborator of a repo, then (in regards to issues)
- you can open issues
- you can comment on all existing issues (open or closed)
- you can close your own issues
- you can re-open your own issues if you closed them yourself
- you cannot close or re-open issues opened by someone else
- you cannot re-open your own issues if a repo collaborator closed them
The situation you report falls in the last case, so you cannot reopen it. You can ask in the issue thread if a repo collaborator can reopen it, or you can open a new issue and reference the old one (by mentioning its number preceded by a hash sign, e.g. #123).