The Articles provides steps to follow while upgrading the device and details to check for if the device does not come up after upgrade.

QFX 5100 became unresponsive after upgrade to 21.xx Version and the following reboot. The device  was recovered by power cycling it.

1. Connect to the device console port using an out-of-band connection. Use an out-of-band connection from the console, because in-band connections are lost during the upgrade process.
2. Always log the console session while device is being upgraded/downgraded.
3. Do not use the no-validate command while upgrading the device as it will not check the configs compatibility with current Junos version being added which was used in our case
4. To find the reason , check for any hardware errors or logs present before upgrade, if there are no issues before upgrade check for console logs if there are any errors seen while device was being upgraded
5. Make sure there is no issue on-site and all Power supplies are working fine
6. Try to power cycle the device if the device does not come up with the help of onsite