Generally one would restore the current/latest firmware. But if the current/latest firmware is causing your My Cloud to brick then one can try using a previous firmware. See the following link which has past firmware links for the first gen (v3.x and v4.x) and second gen (v2.x) single bay/single drive My Cloud units.
What ever firmware version you use, make sure to turn off the automatic firmware updating feature in the My Cloud Dashboard > Settings section.
