I2C Menu displaying problem on LCD 20x4

Hi,
i am new to LPC1768 and I2C as well. My program does the very basic thing that is it shows the menu using I2C on LCD 20x4. My Menu Options are Below:-
>1 PWM 2 Analog
in normal my program operates well. When i click on PWM in normal condition it show me some more menus:-
>1 Frequency 0_ 2 duty cycle 0_

i can change the value of frequency and the duty cycle as well:

but the problem arise when i manually change the value of frequency in program. i change the value of frequecy to 1000 and it gives me error and error is:-

at start when i burn the program on MCU and reset the microcontroller. it shows me first menu that is >1 PWM and Analog and when i want to select the PWM it stucks there and all operation stop untill i reset the microcontroller.
Please help me in finding the solution.
Thanks in advance.