I got the same problem...!!
My board was working fine before a day(in fact, before a few mins) it suddenly stoped uploading.
The TX light is not working after compiling..
I could not find out the problem..!!
*Arduino Uno R3
*Windows 7 , IDE 1.0.4
*(CORRECT PORT, DRIVERs, BOARD have selected.(not sure about the programmer option)