Reinstalling new server

The seed is stored per world. If you reinstall the server, it depends on whether the "Worlds" folder (and the server.properties file) were deleted. The active world is determined by the server.properties file (it's the "World_Name" entry). If that world still exists in the "Worlds" folder, it will still be loaded when starting the server.

If the server.properties file was deleted but the world is still there, just enter the world name there to instruct the server to load it