Java gone missing - but only from Ensemble!

An interface running on Ensemble generates PDFs by building a command line and invoking c:\InterSystems\Ensemble\fop\fop.bat

One day last week the PDF generation stopped working. The output from the .bat script says:

The Server has not restarted, and the Java version has not been updated. Ensemble has not restarted. And Java is still accessible outside of Ensemble from the command line. So how come Ensemble has "lost" java? Any clues as to what might have gone on? We are cautious about restarting Ensemble (which might clear things) without knowing why it happened - thankfully this is a our test server, but this has happened right in the middle of UAT for this interface, and we are nervous about releasing to production if we don't know what it failed...