Hello @wabuckeye,
Thank you for reaching out to the Community and telling us about this issue. I'm sorry to hear you're running into this problem. First, check the account register the data went missing from. Check to make sure it is sorted by date and that no filters are active (checking to see if the data is there, but hidden).
If that doesn't resolve it, next check to see if maybe this is a duplicate account and the missing history is still in Quicken, just in a different account. To check that, go to Tools>Account List. If there are hidden accounts, you will have a checkbox at the lower left giving an option to make those hidden accounts visible. Review the list to see if there are any extra accounts that may contain the missing information.
Thank you.