I have no sectors...?

[Ex]

I was on the sector overlay, all of the sudden sectors are gone so I cycle through overlays, nothing shows up on overlay. My planets and sectors is 100% empty. Hitting create sector on a planet also does literally nothing, what the ♥♥♥♥?

[MrOctober]

I'm assuming this is Vanilla right?
It looks like there's a couple instances of exactly this happening to people. And for almost all of them the only conclusion they could come to was that it's a glitch in your game save, and should be fine if you start another game. Unfortunately that does mean your run may be screwed.
Some times this kinda thing can be fixed by just going back on a save, or restarting the game (which I assume you've done).

[Cryten]

Since october covered a specific scenario I will cover the generic scenario. Uninstall all mods and delete the mod folder in documents. Then verify files. If you have mods that are critical to existing species then you likely cannot do anything. Verifying files will also help against possible corrupted files. Sometimes just saving a game and reloading can work through glitches. Also check to see if you still have a capital as the capital sector is the primary sector and should always exist. Other then that just make sure you are up to date your game is up to date and your chipset and graphics drivers are current (for a given definition of current).