Find your LR 5 catalog (default location is usually Pictures folder)
Double click on your named catalog ending .lrcat to launch
LR5 cannot open a LR6 catalog if that's the situation you're describing.
As far as what to do, you can open your LR5 catalog and import photos that you've edited only in LR6 and continue using LR5 with those photos. If, when using LR6, you saved your metadata to xmp files, then you may be able to import new, LR6-only photos into LR5 and have LR5 interpret as much of the settings as it can, ignoring the new settings that are new to LR6.