it should hum after you plug it in.
the meters fail in these units if it hums it should be working. make sure you plug it in to an outlet that will handle the load.
also if you plugged it in before connecting to the battery you may have tripped the breaker switch some are manual some reset themselves check to see if there is a round black stem type button if yes push it if no try unplugging the unit for a minute.
If no hum maybe bad charger but check outlet for power first.

In addition to the previous answer, make sure the charging clips are making good contact. Sometimes you have to remove the battery cables and clean the connections, as the build up an oxidation that increases the electrical resistance. Make your connections to the battery first then plug in the unit.

One more thing to add, is that this may take all night if the battery is severely drained–one or two hours may not cut it.