Howdy Kathrynhw,
We see that you're looking for help with how to undo deleted text in the Notes app on your iOS device, so let's see how we can help with that.
You may be able to shake the device to undo the previous action depending on the settings that you have enabled or disabled. While we understand that some time has passed, you can still give this a try to see if that action is still the last to be able to undo. If you see that you're not getting the "Undo" option with shaking, let's check the settings to see if this is disabled. Take a look at the two resources below that cover this with both an iPhone as well as an iPad.
Adjust touch settings on iPhone
Adjust touch settings on iPad
You'll want to follow along with the section labeled "Turn off Shake to Undo" for help with locating the setting, but you'll want to be sure this is enabled. If you see that "Undo" doesn't appear, it may be due to the length of time since the last action. If you see that it's not working, you can try a new note just to test and make sure this will work in the future if this happens again.
We hope this is able to help.
Have a great day!