Replaced and initialised HDD. RAID no longer intact

RAID repotted degraded but intact after disk failure.
Bought a new HDD and initialised via console
After restart, all HDD are present but one has dropped out of the original RAID
All and any advice welcomed
Bought a new HDD and initialised via console
After restart, all HDD are present but one has dropped out of the original RAID
All and any advice welcomed