The title says it all. I made some changes & closed/re-opened Delphi, so I Can't use Ctrl-Z to undo my changes.
How do I compare the current file with previous versions?
For code only, if you don't have a version control system installed, you can view recent changes by using the History tab at the bottom of the code editor window, and then click the Differences tab. How far back you can go depends on how you've configured it; the default is 10 revisions. You can change the default in Tools->Options->Editor Options->File backup limit.