Well, then let's try another way. First with the question, if you installed OMV on one of HDDs, or on a separate drive, before you added RAID drives?
In first case it looks like the HDD, you replaced first, was the HDD, which OMV installed on and set as boot drive, and after you replaced it, BIOS did not find bootable HDD no more and changed its Boot Order settings or just cleared them. In second case, even if there was no need to, BIOS seems to have reacted same way.
So next step, I'd do, is to check BIOS Boor Order or simply try Boot Override one by one. And if this does not do it, "Load optimized default settings" or even clear CMOS completely. At least the latter must BIOS let find a bootable drive if any.
If OMV boots again now, then go Web GUI and there directly to Storage->S.M.A.R.T.->Devices and see, which of devices has status "bad" - should be the same as BIOS complained about -, highliight it and then click "Show details" and there "Attributes". Please make screenshot(s) of the bad attribute(s) and post it here. Further look, if OMV already set RAID5 "degraded" or still not, but anyway you should be able to access all your data to make backup(s). After data backup I'd suggest to clone the bad disk to your new one either from within OMV by omv-extras plugin "openmediavault diskclone" or by using a clonezilla boot stick.
If BIOS is not able to boot from any of the disks, then things get more complicated. Because you either need extra tool (kit) booting from an USB stick to repair your original boot drive or install OMV completely new then. In case of new OMV installation I strongly recommend to use a seperate small (32GB is enough) but HiQ SSD for system. You can add existing RAID drive set then and run all procedures needed to exchange one drive via Web GUI.