Check wiring
http://reprap.org/wiki/File:Rampswire14.svgCheck the pots on the drivers
Check the firmware if you have changed something by mistake
Check your Power Supply cables if current is going trough , and if you can supply the proper amps for the motors. 1.2 - 2amps .Like if you are running your PS to the ramps on 1 20 awg wire it wont do it.Use 10-12 for best performance
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P.S. check your ground to the 5amp rail.