First try holding the power button down (15 seconds or so) until the phone reboots. If it still boots into the download screen, you somehow managed to wipe your whole Android system. You'll have to reflash it. See
[Samsung] How to flash Stock ROM via ODIN. (If anything isn't clear,
join Android Central and tell me what is, here in this conversation [you can't do that unless you join, and a post saying "I don't understand" <something> isn't going to be useful because no one would know what you're referring to] and I'll rewrite it so it's clear. If you have a problem with something in that post, others probably do too.)
