System would hang on boot screen with linux515, switched to linux419 and everything is okay. How can I figure out what went wrong?

I applied the stable update this morning and all went well until this evening when there were a few linux419 updates. Once I installed those and rebooted the system would hang on the boot up screen for my motherboard and stay on the screen with the brand name. I booted into the system with a Manjaro USB and tried doing a lot of things before finally figuring out I could switch kernels at boot. As soon as I changed kernel from linux515 to linux419 everything was back to normal. How can I figure out what was causing the issues?

Sounds like you are in between the kernels syncing.
Refresh mirrors/update.

Next time esomething like this happens should I try that first? So would I use pacman-mirrors after chroot in from a USB?

You can run mirrors sorting again - it should attempt to prioritize ones that have synced.
But you likely just needed to run the update again after waiting a bit.

Thank you for your help.