I'm having the same issue. Rooted my KF couple weeks back using KFU, but didn't install TWRP or FFF. Found a rom I wanted to use, went to install TWRP, and the install failed. Ever since then, I can't get ADB and the KF to communicate. The KF works fine otherwise-- I still have my stock/rooted KF with Go Launcher. But I have no way of doing anything else with it right now. To sum up what I've done up to the point of the issue:
- Used KFU to get full root
- Still had free month of Amazon Prime, so I downloaded Voodoo OTA Rootkeeper to temporarily unroot to watch video
- OTA Rootkeeper failed to re-root once (after many times of going back and forth)
- Downloaded KFU 0.9.3, and went full root once again
- Tried to install TWRP, failed.
- Exited KFU with KF still plugged in
- Got caught in boot screen jail (normal KF boot screen- not FFF or triangle)
- Got back into KFU, reset normal boot- everything cool
And that is the last time KFU has been able to see my KF. Ever since that point, ADB does not recognize it.
So what have I done?
Reinstalled all drivers.
Deleted anything on my computer related to ADB, Android, or SDK
(restarting computer on every change)
Tried on Win7 laptop, tried on WinXP laptop.
 changed the names in android_winusb file to anything I could think of, over and over as part of the above attempts
Used my fresh WinXP desktop(no android or ADB or SDK ever installed on it- just wiped and re-installed WinXP)
Hooked up my Samsung Fascinate to the laptops, reinstalled drivers for that, ADB works and recognizes it fine.
Thought maybe it was stuck in fastboot, tried the "fastboot -i 0x1949 oem idme bootmode 4000" because if it's stuck in fastboot, ADB won't see it
fastboot doesn't see it either - got hung at <waiting for device>- tried hard power off/unplug/replug to no avail
Have I missed anything? Is there a way to use a terminal emulator on the KF itself to do some of these commands?