If your raid array is degraded, but you think your HDD is good, you can try back it again. You CAN NOT just change status of failed disk to online, because you server data will be almost lost.

/root/bin/MegaCli64 -pdmarkmissing -physdrv[4:0] -aAll
/root/bin/MegaCli64 -pdprprmv -physdrv[4:0] -a0
/root/bin/MegaCli64 -PDMakeGood -PhysDrv[4:0] -a0
/root/bin/MegaCli64 -PDRbld -Start -PhysDrv[4:0] -a0 or /root/bin/MegaCli64 -pdlocate -start -physdrv[4:0] -a0

Sometimes you need fix foreign configuration:
/root/bin/MegaCli64 -CfgForeign -Clear -aALL
/root/bin/MegaCli64 -PDHSP -Set -PhysDrv [4:0] -a0 or /root/bin/MegaCli64 -PDHSP -Rmv -PhysDrv [4:0] -aN

Make sure you have backups of very important data.