Just happened to me on v8.1.48. I closed designer in the late evening when people opened perspective in the morning it showed "View not found" error. I quickly determined that all page configuration was cleared. I reviewed the audit log and was surprised to find the change was attributed to me as it would seem difficult to mistakenly delete each any every configured page.
I was able to manually restore the page configuration by extracting the gateway backup from the previous day (using 7zip) and examining the configuration contained in
\projects\<project_name>\com.inductiveautomation.perspective\page-config\config.json. I suppose there might be way to restore that file directly instead of the manual method used but I stuck with what I knew in the moment.