Blender 2.8+ is not immune from error, especially those caused unintentionally. One such error is the Missing DNA Block message.
Missing DNA Block Cause
The cause for the Missing DNA Block error seems to relate to the way Blender writes data, or datablocks, to *.blend files as they are being saved or modified, if this process is sufficiently and/or more typically unexpectedly interrupted, for example from Blender crashes, blue-screens or sudden loss of power (power outage), the resulting file will be unusable because its contents are incomplete and malformed. When opened after-the-fact, Blender will display the Missing DNA Block error message in the Status Bar running along the bottom of the application.
Important: due to the Missing DNA Block errors nature it’s generally not possible to simply copy/paste missing file data from functional *.blend or otherwise manually edit files to reinsert whatever might appear to be omitted, largely because the order or organisational location the same block of data might appear may differ with each save or update even where no change or inclusions have been made (no additional data is being included), in other words, the contents of ‘line 20’ may not be the same in both files.
Design note: to double-check the error message that appears briefly in the Status Bar of Blender switch the 3D View to the Info Editor – click the Editor Type selector, then Info from the listed of editors.
Missing DNA Block Append Fix
Depending on the severity of the problem, i.e. how much data is missing, it may be possible to use Append to import content from the broken *.blend file into a clean project. To do this, from the File menu select Append – File » Append – and then browse to the broken file. Click to select and drill-down into the contents then select the items to be imported. Click the Append from Library button top-right.
Design note: providing the user generated data is intact Append should load whatever is selected. If nothing appears in the 3D View despite it being shown available in the File Browser during selection, it may mean Blender has retained references to content that is now missing, corrupt or otherwise unusable.
It may be possible to retrieve user generated content from broken *.blend files that show the Missing DNA Block error but it will depend on the severity of the *.blend files corruption – shown above the File Browser is blank (right arrow) because the user generated content is missing from the file despite filters being set to display it (left arrow).
Rename *.blend1 Fix
More often than not if the Missing DNA Block message appears the affected *.blend file will be utterly unusable, nothing can be done to retrieve the user generated content it otherwise contained. In these instances it may still be possible to reestablish the project checking whether Blender created a *.blend1 backup file in the project folder or directory. Using Windows or other File Explorer (OS/system dependent), if a *.blend1 backup exists duplicate it (right-click, copy then paste) and renamed the file extension as *.blend – remove the ‘1‘ (number one). It should then be possible to successfully open the new file into Blender.
Design note: the amount of retrievable data contained in the .blend1 file will depend on its last saving or updating. Upon opening save a version of the project to create a restore point (avoid saving over the top of the renamed file so it remains intact and a restore point in its own right).