Hi dixxa, welcome to Android Forums.

Have you tried booting into recovery?

I think this is the correct procedure for your phone:

1. Turn off the device.
2. Press and hold the Volume Up and the Volume Down keys and press the power button until the Samsung logo appears.
3. Release the Power button but keep holding the Volume keys.
4. When the Android System Recovery menu appears, release the volume keys.

From there you use the volume keys to move up or down and the power button to select. The first thing I would do is clear the cache and then reboot to see if that fixes it. If not you can go to advanced>clear dalvik cache and then reboot again. Hopefully that will get you back, but if not, you might need to do a full factory reset. That will clear all your apps, user settings and personal databases such as mail and text messages, so you may lose all that.

 

When you perform a factory reset, it will clear the dalvik cache and recompile all the system apps. The long boot should only be the first time.