Go to the project folder, delete the .project and .classpath files and the .settings folder. They contain legacy Eclipse-specific metadata, so Eclipse cannot find your project.
Go back to Eclipse and go to File → New Project ... → Java Project
If it is stored in your workspace, simply enter a new path in the "Name" field; If it was not saved in your workspace, turn off "Use default path" and select the folder where your project is located.
Et voila! Eclipse will generate new metadata, and all your source / binary files will still be present.
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