I recently installed Clockwork Recovery, and used that to install CyanogenMod 10 on my Captivate using this guide. Every thing worked perfectly.
I backed up the stock ROM. Then I realized that my contacts and data were gone in the new ROM. I read in an article that you could restore the data part of the backup using the recovery in Nandroid, so I recovered it. But when I rebooted, the phone got stuck at the spinning CyaogenMod screen.
I made the problem worse by trying to flash the mtkareys zImage kernel, and repeated the process described in the guide. Now, I can't get into recovery mode nor the Android (CyaogenMod) OS.
I suspect that I would need to use some tool that uses my Ubuntu 12.04 or Windows 7 PC, because the only resource that I have access to is download mode in the phone. Nothing else seems to work. Please understand that I am a beginner who has never even written an app for android (I do have the sdk installed though).