It sounds like you are in a boot-loop. Depending on whether you have rooted and modified the system files (based on the way you are describing the issue I am guessing you are not rooted) or not, the actions you take will be different.
If you are not rooted or do not know what rooting is, then follow these instructions to factory reset your device. Otherwise, you will need to either restore a nandroid backup or flash a new rom.
Boot into recovery mode: Pres and hold the power button, home button, and volume up (+) button all at the same time. Once the device starts to boot (you see the first icon on the screen), release the power button only, but continue to press the volume up and home button.
Now, use the volume up and down button to move through the options until you have highlighted factory data reset. Use the power button to "click" the option. Use the volume up/down button to move through the next set of options ( a bunch of "no" and one "yes) to highlight the yes options. Press the power button to select it.
Use the volume up/down button to highlight reboot and power button to select the option. Your device should reboot now.
PLEASE NOTE: This will ERASE all of your personal data and apps from the internal storage (not the SD Card). However, if you are unable to boot into your device to make sure everything is backed up, this is likely the only way to get back into it.
This is not guaranteed to work, but with boot-loops, it is likely your only option.