Did you UNinstall QB2019 or delete any of the QB2019 folders? Or did you just run the QB2020 installer and follow the prompts?
If QB2019 is still installed, when you run it does it open normally with all of your data? And if so, press F2 and in the window that opens, make note of where the data file is located. Look for FILE INFORMATION on the left side about 1/3 of the way down.
The default location for the data file I believe is: C:\Users\Public\Public Documents\Intuit\QuickBooks\Company Files But depending upon when you first started using QBs and if you ever moved it, then then data file could be in any number of places. I would suggest doing a search on your computer for *.QBW. If you've never searched your computer, open This PC (My PC, etc.) and in the upper right you'll see a box with Search this PC. Enter *.QBW and hit Enter. Then wait...
You should find your files and I would then suggest you move the Folder that contains the .QBW file to somewhere where you know they are. Can be in a folder in the Documents or on the C:\ drive (e.g. C:\QuickBooks Data). Anywhere you will know where to look and they will not be accidentally moved or deleted.
Then open QB2020 and in the window that comes up, choose Open Existing Company and then just navigate to where you put the folder. After you open the .QBW file, QBs will remember where that data file is located.
If for some reason the .QBW file was deleted. Do the search for *.QBB and see if you can find the Backup files. You can open QB2020 and tell it to Restore from Backup too.