I'm sorry to hear about that. But you can try to fix this issue manually.
Here are some methods for you to fix your SD card not mounting on your Mac.
Method 1: Insert your SD card into another computer. Try to insert your SD card into another computer to check whether your SD card can work normally.
Method 2: Restart your computer. Shut down your Mac and remove the power cable. Wait for a few latter and turn it back, then replug your SD card to check.
Method 3: Check the format of your SD card. Check whether your SD card has been locked by BitLocker or other software.
Method 4: Update software on Mac. Check whether there is any new version of macOS that can be downloaded. The latest version of macOS is macOS Sonoma (14). Make sure you have upgraded to macOS Sonoma and then try to mount your SD card again.
Method 5: Run First Aid. You can run First Aid in Disk Utility to check and repair the corrupted system files. Once it detects corrupted files, it will repair them automatically.
Method 6: Recover data before formatting. If your SD card still can't be mounted on your iMac, the first thing you need to do is to recover your data from the SD card. You can use iBoysoft Data Recovery for Mac to get back all the data.
Method 7: Format your SD card. After you recover your data from your SD card, you can try to format it. If you worry that some incorrect operations will damage your SD card, you can send it to a local professional to repair or replace it with a new one.