Can not download Pictures from Iphone to computer when connected?

The simplest way you can do to download your photos from your iPhone to your Windows PC is by using a USB cable that came with your device. 

1. You need to unlock you iPhone because your Windows PC won't be able to find your device if it is unlocked. 
2. Connect your iPhone to your PC with the USB cable. 
3. You might see a prompt on your iPhone asking you to Trust This Computer. Tap Trust or Allow to continue. 
4. On your PC, open Photos app. 
5. Select Import. You can then select the photos you want to import and select where you want to save them. Wait for a few seconds before the transfer starts. 

Hope it helps! 

If it's Windows as said in the description, there's a known bug with "Device disconnected" error during transfer, happens due to HEIC > JPEG conversions issues. Also this can happen due to 4k/1080 videos transfer bug. Try changing export settings to "Keep originals" at the bottom of Settings > Photos on the phone. If won't help, get smth like iMazing, never had any issues with file transfer.

Please could you provide more information? For example:

• The steps you are trying to download the photos.
• Any error messages that appear.