I'm not sure what the old version of MD was. I think was 2011, although every time I'd gotten a notice that there was an upgrade available, I've upgraded. The version I upgraded to yesterday (when all my data disappeared) is 2019.1 (1855). I am running macOS Mojave Version 10.14.2.
I also have the MD App on my iPad and my data from the previous version is still there, back to February 2018, when I installed the app. I assume that my backup file feeding the app is in my Dropbox account, but I'm afraid to open that and lose that too.
The Moneydance program and the Moneydance data are stored separately. Upgrading the software on its own does not impact the data.
It sounds like you may have created or be opening a different Data Set. I f you choose the Menu item File --> Open a list of Data Sets that are in the default location should be presented for you to choose from and that you can look at.
If you may have moved your data sets you can search your hard disk for names ending in .moneydance or perhaps .md
Also if you are storing either your data file or your backups on Dropbox that may be an issue going forward. It can also take significant time to open or close the program and is HIGHLY not recommended by the developer.
The newer versions of MD warn against this.
If you are updating from MD 2011 there can be issues and things may need to be updated through successive versions that you have not used. Some have had success updating from 2011->2012->2014->2015->2017 before updating to MD 2019. Upon updating from a previous version MD will keep the old file as a backup file. Also older 2011 datafiles have an extension of .md and newer ones use .moneydance if you use a search function to look for them.