It must be extremely annoying to deal with this kind of problem cause you can't access any of your SD card files on your Windows PC. But don't worry, you can fix it by either allowing your computer to access your phone data, or unlocking the lock screen of your Android phone.
Is this the first time you connect your Android phone to your Windows using USB and try to access your SD card files there? If it does, then this problem may occur because you haven't "Allow this computer to access your phone data".
If it is not the first time you connect your phone to the PC, then should also check your smartphone when connecting it to your PC as it may ask if you want to grant permission for the PC to access the phone. When asked, make sure to tap on the ALLOW button on your Android phone.
Is your Android phone locked when connected to your Windows PC? You should unlock the lock screen of your Android phone (enter the password if necessary), otherwise, you cannot access the phone data via the USB connection. This is a security measure that ensures someone who doesn't know the password of your phone can't access your data through the USB connection.
Moreover, if after allowing your computer to access your phone data and unlocking the Android phone, still the SD card is not showing up on your windows computer and your phone can't even be detected, then you can try to connect your phone to another USB port.
If all these methods don't work, you should consider unplugging your SD card from your Android phone, and connecting it to the PC with an SD card reader.