This index kb explains all the various prompts that an EX switch might end up at when it fails to load Junos and provides links on how to correct that situation.
Can't get to the Loader Prompt (loader>)
Loader Prompt ( loader >)
Debug Prompt (db>)
UBoot Prompt (=>)
Can't load kernel error
'Loading Junos' reported on LCD panel
EX switch – Troubleshoot upgrade failure/crash – System stuck on Boot-up
https://kb.juniper.net/InfoCenter/index?page=content&id=KB20464