I had to move from PSE9 on a Windows 7 computer to PSE2020 on a Windows 10 computer. I backed up my catalog with PSE9, installed PSE2020 on the new computer, and restored the catalog. Most things came through OK, including the tags and file structure. Photos that were in stacks in the old system are still marked as stacks but the Stacks command is grayed out and I can't figure out how to expand the stacks. I know all the stacked photos are in the backup file because I checked. I uninstalled and reinstalled PSE2020. I deleted and re-restored the backup file into a new catalog. I optimized and repaired the new catalog and rebuilt the visual similarity index. I still can't expand the stacks. Help!
I think yoiu are probably in the "adaptive" view of the Organizer (which is the default that trips up many). Press Ctrl+D to get into the Details view and your stacks should become visible again. If this is not the case, please give us a screenshot of what you are seeing.