RAID5 fail when one harddrive fail

hi
i have a problem my server not booting when in one of hard drive fail in RAID5
i have 4 physical drive
my HP server pro DL160 G5 have 750 GB 4 drive RAID5 configure
last week my server suddely restart and then one drive fail in RAID 5 but server not booting after restart and array show my all logical drive fail.
in this my company imp data so how can i get my data
if i change hard drive can it work fine and rebuild array and no lost of my data or something i do for saving my comp data help me pls.