I took a great selfie at the Space Needle, but I cut off my head. How can I uncrop to get my head in the pic? Can MS Paint or Photoshop do that?
The short answer is no, a photo is just some recorded data and if you've only recorded 2/3rds of your head, you never had the data for that last third so there's no way to get it back.
The long answer is maybe. Photoshop CC and Lightroom contain a tool called content aware that can estimate what is missing in a photo and fill this. Search on youtube for "extend an image with content aware" and you'll find a number of results showing how you can potentially create more of an image you have taken. This will work great for extending certain aspects of your selfie (namely sky) but will struggle with precise details like your head and the space needle, creating a weird blur instead. At this point, you could always bring in a previous selfie to add the cropped parts of your head. There are plenty of retouching tutorials online that can show you how to make the added part of your photo blend realistically...after some practice.
But the long answer is time consuming, associated with a cost, and potentially going to end with a photo that looks fake & photoshopped. My advice is embrace the crop you accidently set yourself instead!
From your question I'm guessing you accidentally cropped your head out of a photograph, and your head actually is in the original photo. If that's not the case then there's not much we can do to help.
Possibly the original image will still be on whatever device you used to take the picture. Many image editing programs will save a new copy of the picture when you make edits like cropping.
Without knowing more about what device you used to take the picture, and the program you used to crop it, the help I can give is going to be non-specific.
If you used a camera - the original photo will still be on there unless you've deleted it. So you can just import it to your computer again and start afresh.
If you used a phone and you edited the photo on your phone - this will depend on the program you used to do the edit. Try going into the gallery and seeing if the original is still there.