Refrigerator is unplugged a couple days for cleaning, once plugged back in will not start
If the refrigerator was unplugged during its defrost cycle, it may take some time to complete the cycle before kicking the compressor back on. Leave it plugged in for a few hours.

As mentioned in another answer, first check to make sure the outlet has power. Other than that, give it some time and see if it comes to life.