Remember, if you have an iPad or Mac and Share Across Devices is enabled, Do Not Disturb will activate on your iPhone if you enable it on one of your other devices.
Try checking if you have a schedule set up. Go to:
Settings > Focus > Do Not Disturb > scroll down to Set A Schedule
Check if you have an active schedule, tap and select Delete Schedule.
You can also check if you have an automation set up in the Shortcuts app. Open the Shortcuts app, and select Automation in the bottom menu. Check if there is an active automation - if so, swipe to delete it.
Last but not least. If none of the above situations are the case, a restart can often solve these kinds of strange problems. It's worth a try.