Michael,
The re-installation should not matter. Your registered software should work fine. The project itself has not changed.
However, in my experience when a full install is done a few settings can change back to defaults. Try opening the Tools | Options dialog (or use control-F9) with your project open and work your way through the settings. See if anything has obviously changed, or looks odd.
Then try diagram properties for your sequence diagram (or F5). Once again, look for visibility settings and such.
[EDIT: If nothing works to this point, you might try finding and clearing the EA registry settings. I believe some are general and others are user-specific. EA will likely recreate them, but if not a reinstall will certainly do so.
Sparxians (or whoever) please jump in with confirmation or a workaround for this.]
Finally, if you are printing reports, look at the templates. Templates are somewhat like reference data, and might have been reset. This is the important place to look for reports, but the earlier settings can block changes you make here, so do them first.
Last but not least, make sure that the build of EA was not the issue. Build 816 was released at the end of last week, so you might have a 'versioning' issue. If your registered version is build 816, look through the download site and find a kit for build 815 (or whatever build your trial was). See if the earlier build works.
If the earlier build does work, or if none of the above things help and nobody posts a quick answer here, please submit a bug report on this issue. Search my recent posts for a best practice to link to this thread.
Please let us know how this goes regardless.
David