help with fpga clock?

i'm using a spartan 3an starter kit. the board comes with a 50 MHz clock but i want a 40 MHz clock. so
i changed the constraints and the program generated this line:
NET "clock" TNM_NET = clock;
TIMESPEC TS_clock = PERIOD "clock" 25 ns HIGH 50%;
no errors occur but when i run this program on the board it does nothing however if i use the 50MHz clock it works fine.  What could be wrong?