iphone 3gs does not start

i wonder if anyone can help me, my iphone 3GS has been crashing randomly over the last couple of days, usually when i receive calls, texts or emails. usually i have been able to reboot it, but two hours ago it crashed again and now when i try to start it again i only get the apple logo and after about 10 seconds it turns off again. i tried resetting it, won't work. my macbook does not recognize my iphone when i plug it in. anyone knows what to do? this phone is my lifeline