I was on Facebook on my phone and it wouldn't load - causing my phone to become really slow - so I turned it off (this usually works) and when I turned it back on again it won't go past a flashing Samsung screen. I have taken the battery out and left it out then put it back in but my phone still won't work
I have heard you can do something too reboot the system, but I reaallyyy don't want to do that because I would loose all of my apps, memory and everything, which I need!
Please help I really need my phone!

Honestly... this is why you back your data up... it happens far more often than you think.

You might not have a choice, hopefully someone has an Idea... Being a Samsung I assume its got the Android O/S.

Sounds like something other than Facebook caused your phone to crap out. Could be a virus. Did you have any sort of AV on the phone? Don't know if this will make a difference, but you may try booting up the phone without the battery while plugged in to a charger.