Itunes update and iphone update erased everything 3G

so i just downloaded the new itunes and installed the new iphone update. As it was done downloading, my phone shut off and it restarted. It then has an icon of a itunes cord and an itunes logo. i look on my itunes and it says iphone n/a.and that i need to restore my iphone. Why did it erase everything? now i cant even restore from backup.how do i stop this in the future? this is the second time in 2 months that this problem has occured.really annoying considering i lost everything. someone pleas help.
iphone 3g iPhone OS 3.1.2