The 1080p resolution no longer works. Why?

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today despite this Elvira

May I know if you already tried power cycling the console?
1. Turn off your console by pressing the Xbox button on the front of the console for about 10 seconds, until it shuts down completely.
2. Unplug the console's power cable from the console and from the power outlet. Wait 5 minutes.
Important: Be sure to wait at least 5 minutes. This step resets the power supply.
3. Plug the console power cable back in.
4. Press the Xbox button on your console to turn it on.

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If that doesn't work, may I know if you tried connecting the console to a different HDMI port?
Or try using a different HDMI cord

Thank you, in this case I can advise you one more thing:
Perform an offline update for your console. In that way Xbox system will be installed from scratch.
For this you need a USB and PC to format it and transfer files provided in the link below (with detailed steps and requirements):

https://support.xbox.com/help/hardware-network/...

PS: if you're using an insider build / beta OS in your console you need to leave the insider program first
If not, the offline update is not going to work

It annoys me that it works perfectly on 720p but when I change it to 1080p it doesn't work anymore. And it worked perfectly before.

Just to confirm, did you already tried using a different HDMI cord or connecting the console to a different HDMI port in your TV?

I turned it on again now and again it doesn't work at 1080p. I don't know what else to do.