Nothing works. My bad! (4)
I believe the answer is:
oops
'my bad' is the definition.
(I've seen this before)
'nothing works' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'works' becomes 'ops' (op is short for opus, a work).
'o'+'ops'='OOPS'
(Other definitions for oops that I've seen before include "Oh dear, that's a mistake!" , "Oh dear, I've spilt something!" , "Exclamation on making obvious mistake" , "Exclamation of surprise or apology when making mistake" , "I've slipped" .)