Same here. Adobe please answer this question.
Unless you removed LR 5.7, it should still be on your computer and you can still use it; however the catalogs for LR 6 or LR CC are not backward compatible and cannot be used in LR 5. Any new imports into LR 6 will not appear in LR 5 and will have to be imported again. Also any develop changes made in LR 6 will not appear in LR 5.
FYI, this a users forum with members helping other members, We are not Adobe. Occasionally, a member of the Adobe staff will monitor this form and may respond from time to time.
Select all the photos you have changed recently, maybe using a smart collection, and use Ctrl S (Win) or Cmd S (Mac) to save most of your work back to the photos.
Now open your 5.7 catalogue and select the images again. Then use Metadata > Read Metadata from File.
Not all Lr6 work will be brought into 5.7, but most of it.
LR 5.x.x, whichever version you had installed before you upgraded to LR 6, is still on your computer and fully functional. That is unless you removed it, uninstalled it, yourself after installing LR 6. LR 6 does not remove, uninstall, LR 5.
Look in your Applications folder, Mac, or the Start menu, Windows, for LR 5. If on Windows and you don't find a link to LR 5 in the start menu open Win/File Explorer/My Computer and look in C:\Program File\Adobe for it and create a shortcut to the Lightroom.exe file on your desktop.
No you can't use the Lightroom 6 catalog in LR 5 but then as with LR 5 the program your LR 5 catalog is still on your hard drive (or wherever it was prior to upgrading to LR6). That is unless you Deleted it manually yourself.
Any work you have done in LR 6 will need to be Redone in LR 5 including importing all images you have taken since upgrading to LR 6 into the LR 5 catalog file.