1st:
First, make sure that you’ve got new batteries replaced. It takes very little electricity to show the display on the remote controller, doesn’t mean the batteries are OK. The batteries may be too weak to send the signal across to the air conditioner.

2nd :
The same brand of air conditioner usually have more than 2 different  types of remote controllers. If you got them mixed up, they don’t work.

Troubleshooting:
Most air conditioners have a manual on/off button on the air conditioners. Most of them are inside the flap cover. Open the flap first:

Usually, the ON /OFF button is on the right side of the air conditioner:

Press the small little button, you may need to use a pen to press it. Does it work?

Yes: Either the remote controller is spoilt (test it on another air conditioner) or the infra-red signal receiver is faulty.

No : Either there’s no power supply to the unit (check your circuit breaker) or there’s a problem with your air conditioner’s electronic components or electrical cables.