Hey Designer! Wassup. I assume you to be a designer working with photoshop. Else you would not have been here. Anyway, let’s get to the point straightaway. You are here because you
Can not edit a PNG file in photoshop or probably your background layer is locked.
Further, you are not able to create any layers using marquee tools or change colors using the change hue saturation options. Probable all major functions are disabled as well. You might be seeing a lock icon on your main image that is now the background layer.
If you have the problems in editing PNG file as mentioned above try these simple steps and you would be doing good in no time.
1. Double click the lock icon. If it unlocks then you can try editing and see if everything is all right. If no try the next step.
2.Duplicate the layer. Hide the old layer. You now have a top level editable layer that is the only one visible. This maintains transparency and all the other features of the png without trying to convert the image. (Suggested by Jared Kreft via comments)
3. Try to save the PNG image or whatever you are working on to jpeg format. Now open it in photoshop and try to edit. Working ? No? then try the next step.
4. Try to edit the color scheme to RGB.
You may also try some online tools to edit PNG files. If you do not have Photoshop or are in a hurry to edit an image try some of these tools.