This never happened to me in the past. But I recently upgraded to OS 10.11.6 and from LR 5 to LR Classic CC 7.2 and now it happens about once each session. I have to shut down LR and re-open it to bring back the side panels.
Very annoying when this still happens, without having been corrected, to what is as of yesterday, the "old" versions of both the system and Classic CC.
Brad