Occasionally, for some reason I haven't been able to figure out and therefore can't reliably replicate, all of Lightroom's user interface panes except the center one disappear. . . . The only way I've found to restore the display to its normal state is to kill and relaunch the Lightroom process.
It took four months, but Adobe today released a new revision of Lightroom
with one or possibly more fixes that have "made progress" in resolving this bug.
(I wish Adobe would be somewhat more open about reporting interim progress regarding verified problems and describing the known cause(s)—especially in cases like this, where it appears the problem has not been completely resolved. Aside from the benefit in customer relations, a little more transparency would help technically-knowledgeable end-users who encounter residual problems to replicate them reliably, and thus provide Adobe with more useful follow-up information. I do give Adobe credit, however, for at least making available a public Beta Test version of this release to volunteers who were experiencing the problem frequently.)