Samsung Captivate not booting after root
What Izzy said is correct. Also, never give up! I found that there is always something you can do to unbrick your captivate. 

The xda-developers wiki page is very useful in this regard. The sections about bricking and unbricking should be very useful. While it's very difficult to enumerate all possible ways of unbricking here, understanding the nature of the brick (soft or hard) and looking for alternative ways in the forums is always helpful. The techniques that I know of so far are: 


The 3-button method and its variations, as listed in the wiki page above. (There are variations, just search for them in xda-developers)
Using your PC and the USB cable. Izzy's answer is one such method. 
Hardware solutions that involve cutting a USB cable or using resistors. When you have a hard brick, you may have to resort to these. 


Good luck and don't stop searching! There is always a way to unbrick your Captivate.