SD card is not showing up, what should I do?

How to fix SD card not showing up in Windows:

Check the connection, including the SD card reader or adapter, USB port, and the SD card a lot if your PC has one.

Update or reinstall the SD card driver in Device Manager. Navigate to Control Panel > System > Device Manager > Disk drives > right-click on your SD card > Update Driver. 

Assign a new drive letter to your SD card from Disk Management > right-click on your SD card > Change Drive Letter and Paths > Change > Assign the following drive letter.

Run CHKDSK command to repair a corrupted SD card.

Format the SD card in Disk Management. However, to recover data from corrupted SD card, you should first use iBoysoft Data Recovery for Windows before proceeding with the formatting.

How to fix SD card not showing up on Mac:

Check the Finder and desktop preferences. Go to Finder > Preferences > General and ensure that the external disks option is ticked. Under Sidebar, apply the same setting.

Unlock the SD card if the SD card is write-protected. By adjusting the lock slider on your SD card, you can work on it again.

If the SD card reader is not working, replace it with a new one.

Go to Finder > Applications > Utilities > Disk Utility, can you see your SD card there? If it's not mounted, manually mount it to make it work. If you can't, possibly, your SD card is having some hardware issues.

Reset NVRAM/PRAM. You should switch off your Mac, then turn it back on while pressing and holding Option + Command + P + R to reset the NVRAM. Hold those keys down for around 20 seconds. Release the keys after that to let your Mac restart properly.

Repair a problematic SD card with First Aid in Disk Utility.

Format the SD card in Disk Utility. Similarly, you should run iBoysoft Data Recovery for Mac to recover data from corrupted SD card first. Then format the SD card as your last resort.