"Only one top level element is allowed in an XML document"
There is the requirement for XML document to have exactly one root element.
The error informs that ".dproj file" which is the document in XML format does not adhere to this requirement and thus malformed.
Perhaps, the .dproj file has been corrupted.
The most expedient way to solve this issue is to remove the .dproj file, launch the correspondent .dpr file and let the IDE regenerate one.
If the matter is with dpk file, the way to solve is to create an empty package and import the files in.