Exclamation point & song not copying onto Iphone

Hey guys

So from one day to the next my iphone 4s has "no music content" on it in the music folder. So I go on Itunes and try to sync my songs back onto my iphone but it doesn't work, which is strange because it used to work perfectly. While I have my phone plugged into my computer, I go onto my Iphone tab in itunes and then I go onto "On This Iphone" and all my songs are greyed out. I do some research and they tell me to check the "manually manage songs and videos" box. So that's what I do but it doesn't do anything, and when I try to autofill my songs onto my iphone it tells me to change the bit rate to 128 aac. I do that but still nothing. So instead I uncheck the manually manage songs and keep the convert bit rate to 128 aac. Then only 335 out of my 995 songs sync onto my iphone.. the rest have exlamation points next to them. What do I do now? They used too ALL be on my phone, how is this possible to happen one day to the next?! PLEASE HELP.