M4300 lost config during upgrade

I can't reliably tell you since I'm not from Netgear but I might be able to give you a "good" guess. I think the firmware image is rather a completely separate file or partition (later shows it's more likely a file) and the bootloader has a saved value on what image to load as operational code.

Unlike M4100 (only has 'dir usb'), the M4300's have a 'dir' command that can show the flash's content like this:

(m4300-12.0.7.12) #dir

         1  drwx                 3192 May 08 2019 15:01:14 .
        44  drwx                    0 Feb 01 2019 07:14:19 ..
       603  -rw-                   96 Jan 01 1970 00:01:40 snmpOprData.cfg
       924  -rw-                 6839 May 08 2019 15:01:14 running-config
        91  -rw-                  156 Jan 04 1970 13:29:22 dh512.pem
       752  -rw-             37630117 Jan 22 1970 14:21:56 image1
       753  -rw-             37710793 Jan 22 1970 14:26:16 image2
[...]
       923  -rw-                 6839 May 08 2019 15:01:09 startup-config
[...]
Total Size: 229474304
Bytes Free: 157040640

Try to check how it looks like on your stack. If the startup-config is still here it might not have been loaded properly or it might be corrupt. Unfortunately I remember that most Netgear switches would only show error such messages over the serial console during bootup but it won't end up in the log buffer.

You could try to use the copy command to get the file to a USB key or i.e. SCP it to another system.

I'd likely first try this before trying to reboot th stack into the latest or the previous firmware to see if the loading of the startup-config fails.

In contrast to the M4100's (that I have one in my lab) , it might be possible that listing the file system and copying the startup-config through ZMODEM at the boot menu might have been removed when comparing this article from Netgear about password recovery:
https://kb.netgear.com/30326/M4300-How-do-I-recover-the-password-using-the-boot-menu