My computer (dell optiplex g280) with xp sp2 has been working well all along with smadav 2017 running on it. I inserted a memory and discovered that my smadav no longer launch and not able to do USB auto scan, but the pc working perfectly. I removed the hard disk this morning to attach to and scan on other healthy PC, but it was not recorgnized by the PC. I resit the data and power cables but to no avail. I later returned it to its original PC, still it could not be detected: in BIOS it write not present and could not detect bootable device in normal booting with a "no bootable device not found" error. I have powered on same system this morning twice prior to removal of the said hard disk. Majority of my works are domiciled on the hard disk. So tech experts and engineers, I need your expert advice because I have been feeling unwell over this as it the only means to my livelihood. Thanks in advance

Please I am in need of your advice on what to do to prevent lose of my data. Once again, thanks in advance