The following are possible scenarios where this can occur:
(1) Usually, this indicates that the PROG_B or INIT_B pins are pulled down to GND.
(2) Check your power rails. Vccint or Vccaux might not be at the correct level. Verify this against the data sheet for the specific device.
(3) Check the Security Option set in Bitgen. Some security options prohibit readback. For more information, refer to (Xilinx Answer 30883)
(4) If you see this on Virtex-6 FPGA in ISE 12.1, 12.2 or 12.3 design tools, refer to (Xilinx Answer 39566)
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