Thank you for reaching out to us.
From the description mentioned I could understand that you are having issue accessing the switch after upgrade.
Could you help me with Switch model and switch firmware version upgraded to ?
Also before upgrade could you let us know the old firmware that was on the switch ?
Do you have access to switch using console ?
I see that you have mentioned "I have also tried some of the upgrade steps but PuTTY can’t locate the switch." could you brief us a little more on how you tried upgrading the switch when putty was unable to locate the switch
I have mentioned a link below using which you can try recovering the switch using boot mode. See if that helps.