Wondering if you have a back-up of the music files, so you can restore them.
Apple generally advises us to back up everything before updating / upgrading / the operating system.
If the iPhone was short of memory / storage space after the update, it may have been forced to delete something old / replaceable / redownloadable / in order to keep operating, and do all you ask it to.
Eg: An iPhone generally needs around 8Gb free, in order to continue operating smoothly:
See: How to check the storage on your iPhone and iPad - Apple Support
If you find more space is needed: See: How to back up - Apple Support
See: "Manage space on your iPhone in the article below for tips:
If you need more space for an update on your iPhone, iPad or iPod touch - Apple Support
Else: Try a search for one of the CD music files to see if they are actually still on iPhone
If the music was being synced to iPhone from Mac or iCloud, try checking the settings are correct.
If needed:
Music Support Website: (info. / support / contact details etc.)
(There's a "get Support" link (online chat / phone / email etc.) at the bottom of the webpage below:)
Music - Official Apple Support