The same thing happened to my OnePlus Nord N10. It got stuck at the bootloader screen after I upgraded from Ubuntu Touch 20.04 to 24.04.
I solved it as @MrT10001 suggested:
- Started Ubuntu Touch in recovery mode
- Enabled Fastboot and restarted the devices.
- Installed Ubuntu Touch 24.04 via the installer desktop application.
After that, the problem was fixed, and my device is now running 24.04 without any problems.