You are in U-boot prompt. I think you pressed SPACE key during autoboot and hence you are in that prompt. Reboot the device and boot into loader prompt and then try to recover from USB:
Reboot the device.
The following message appears:
Loading /boot/defaults/loader.conf
After this message appears, you see the following prompt:
Hit [Enter] to boot immediately, or space bar for command prompt.
Press the space bar to access the loader prompt (loader>).
The loader> prompt appears. Enter:
loader> install file:///<image-path-on-usb>
An example of a command is as follows:
Reference:
https://www.juniper.net/documentation/en_US/junos12.1x44/topics/task/installation/security-junos-os-boot-loader-usb-storage-device-srx-series-device-installing.html
https://www.juniper.net/documentation/en_US/junos/topics/task/installation/security-junos-os-tftp-srx-series-device-installing.html