AC doesn't work after I changed batteries

Do the easy stuff first! Is this a new install or has it been working well for a time and now it is not? Is the entire unit continuing to run or just the fan? Thermostats generally turn the unit on and off when the desired temp is reached but the fan itself (not the compressor or heat strips) typically operates in 3 modes, off, on or auto. check the fan setting.

I can only suggest that you check and make sure all of the pins are straight and not bent over from trying to align the back plate with the back of the thermostat. Ive done it and its easy to do if you are not very carefull.

carefully pull out on the bottom front of thermostat

Remove thermostat from wall, leaving base plate on wall.
Model number appears on back of device.
Google search thermostat model in following format: Honeywell xxxx32 thermostat .pdf
Adding .pdf to search parameter will yield product manual.
Also while thermostat is off the wall, change batteries located in compartment on back of thermostat. These batteries must be changed periodically, for example each 3-4 years.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Add a comment and describe brand, type and model of product. And what this product does. Is this a thermostat?
Also say if this is new installation or older installation
If you have Honeywell thermostat, then model number and AA battery are located on back of device. Replace battery.
Web search your model number and include .pdf in search parameter to obtain product manual, Use Ctrl+f to word-search manual for troubleshoot section. And view manual for illustrations showing location of battery.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Change battery located on back of thermostat??

Go to google
Search for your model number 1234XX .pdf
Add .pdf in search parameter.
Honeywell posts most their thermostat manuals.
Open pdf file, and then Use Ctrl+F for search feature
Word-Search manual

To find model number on thermostat, remove from wall.
Model number appears on back.
Don't forget to replace battery while you have back exposed.
Dead battery can also cause lockout and malfunction

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Check battery's , & try inturepting power for 5 to 10 min,if no response
change ,24 volt transformer or control .

Susan, the "90" series of thermostats covers a couple of different thermostats made by White-Rodgers. Here's how to find your model number. With that info, I can check the manual specific to your thermostat.

In the mean time, check the display for the word BATTERY or BAT. (Flashing) on the display. Battery flashing or blinking on the digital display indicates low batteries. If the batteries have recently been changed and the thermostat continues to indicate low batteries, use a pencil eraser to polish each of the battery contacts inside the back of the thermostat. Do not bend them up and down when polishing or you might break them. When complete, they should all be as shiny as a mirror. This will assure proper battery back-up, allow the backlight to work correctly and remove the flashing battery symbol from the screen. Battery (Solid) on Display The battery symbol (not blinking) indicates a loss of power (24 volts) from the heating and cooling equipment to the thermostat. Normally the heating and cooling do not operate when the battery indicator is on. If the battery indicator is on, check the fuses or breakers, make sure the furnace is switched on, and verify the blower compartment door is closed properly. The battery display is designed to indicate the thermostat is running on batteries, so replacing batteries has no effect on the indicator. See No Heat/No Cool for more information. Note: Some heat only systems (zoned systems) disconnect power from the thermostat during a normal heating cycle and display the battery symbol. Millivolt heating systems always indicate battery because they do not supply 24 volts to the thermostat.

A beep often is provided as audible feedback when keys are depressed on many thermostats. You should check to make sure that no key is stuck in the "in" position or is otherwise activated.