More information could be helpful. For example...
1. Which version of SQL Developer? JDK version? Operating system?
2. Provide a sample of the SQL contained in the reopened .sql file. The file extension is .sql, isn't it?
3. Have you disabled any of the product features under Tools > Features? Whether yes or no, try hitting the Clear Cache button.
4. Have you installed any third party extensions?
5. If running the 4.1.1 version, have you checked which SQL statements get executed via the Log panel's Statements tab?
6. Have you tried launching SQL Developer from a command line console using bin\sqldeveloper.exe?
(Perhaps some errors will appear in the console, or even in the regular Logging panel's Messages log or Logging page.)
The Query Builder feature should be able to parse the SQL to make it editable in the Builder, even if no database connection is active.