Keil can't find my TM4c1294gxl

The IDE is Keil uVision4,and it says ,"could not initialize the target device. please power cycle the board and try again" and "flash download failed target DLL cancelled".

I programmed it initially with a blinky program using gpio port pn0 and pn1 connected to on-board leds. Initially the program run but now its showing this error. 

Am unable to find out the problem! 

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