*HELP* I think my iPhone just died *HELP*

For the past few weeks my iPhone 3G S has been acting real buggy (apps opening slowly, delays when typing, voicemail ringtone going off when I don't have any voicemails) and a few minutes ago it just died on me! I tried rebooting it by holding down both the button on the top right corner and the button on the lower part of the screen at the same time but all I got was a black screen with the Apple logo in the middle. Now when I try rebooting it again...the screen goes blank for a second and then back to the black screen with the Apple logo in the middle. I've tried charging it, I've tried syncing it (my laptop/iTunes won't even acknowledge it's presence at all now) I've taken out the sim card and replaced it...but nothing seems to work! I hope someone here has some good advice for me becaue I'm starting to panic...I can't afford a brand new iPhone and the nearest Apple store is really far from me.