Make sure you use the correct files and tool for your exact device build and software.
Also a side-note, the chance is small but you can mess up your device (never happened to me and I have jailbroken like 12 devices so far, but can never be sure)

Oh btw, if you do somehow mess up something just hold the power button for 10 seconds, then the power+home button for 10 seconds without letting go of the power button. After that let go of the power button but keep holding the home button for another 10 seconds. This just force-reboots your device