I did an update on my phone and all my photos vanished! How do I retrieve them?

Hi! In order to understand the reasons of this behaviour I suggest to go through the informations provided in this official article by Apple:

Get help with missing photos

https://support.apple.com/en-ie/HT205912

The point is to understand if your photos were stored in iCloud Photo Library or in iCloud Photo Stream or in iTunes (in this case the original photos are still in the Mac used to sync the photos thumbnails on your device)

It will be useful also to know if you made a backup before to update as suggested from Apple:

Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-ie/HT204204

If you want to receive help performing this steps or just want to know more about the features contact Apple Care Official Support and they can guide you verifying if there is any possibility to recover it.

Kind Regards

Your itunes or icloud backup.

Or sync them back via icloud.