Does your printer control board have a reset button? Try pressing the reset button when the printer is turned on (with the USB cable connected), and see if Pronterface can then connect. One of my older control boards (gen6.d) had a similar behavior where I could not connect to the board immediately after turning on the power, but after pressing the reset button, I could connect. [
reprap.org]
Also, did you try rebooting your PC? If you repeatedly plug and unplug printer's USB cable, sometimes the connection port can change (on Linux, the default is /dev/ttyUSB0, but it can change to /dev/ttyUSB1 or /dev/ttyUSB2 if you repeatedly unplug and re-plug the printer's USB cable). By rebooting your PC you reset the connection port back to the default.
If that doesn't work, then you may have to buy a new control board for your printer.