Is the machine turned on?
Did you warm up the machine? (at the start of the day or after 1 hour of not running the machine)
Is the correct milling bit in the machine?
Is the milling bit sticking out long enough?
Is the material in the correct location? (And - if necessary - is it screwed down?)
Is the Z-plate cleared from chips or dust?
Did you calibrate / pull back the bridge?
Is your toolpath saved on dropbox?
Did you pray to the cnc gods?