I changed the batteries and now there is nothing displaying on the screen. The air conditioner is working.

I believe this thermostat will display -ac if it loses power from the furnace or air handler. Your thermostat will operate on low voltage(same as your door bell) and constantly powers your thermostat. I would check your furnace breaker, and fuses to see if there is power to the equipment

According to the owners manual - -
To turn on the screen, push the "SETTING", trip
control, air conditioner (except rear window defroster
button) or audio button.

There's nothing in the book about special instructions should the battery be disconnected, so if the screen will not come on I would guess a fuse or something happened to the power source.

If its new return it. Is the back of the unit clear of obstacles, nothing blocking the air flow.

Before buying new one, try with this step. Remove batteries from your remote and reset it after 5 minutes. And also remove DC wires plugged to indoor sensor display, and after 5 minutes re set it again. Now start your unit. Sometimes it will work out. Thanks.

1.restart device from cable.2.remove battery from remote cont.and try again

Replace electronic control board

Yes, the battery must be replaced. Gently lift or pull of cover, the battery compartment is there. Best of wishes.

The remote stopped working. So I changed the batteries Then remote signal's are OK But Indor Unit is not controlled by remote.

I've checked the webpage, and you can find your answer on Meijer:Haier 7,800 Energy star!

No LCD display would suggest bad battery, incorrect battery install, or unit not seated. Fan may not need LCD to display.