You mention Illustrator in the tags, maybe you saved an image from illustrator? If Illustrator is set to CMYK, it will output CMYK files whenever possible, and opening these files in photoshop will put photoshop in that mode as well. Use document color more in the file menu to change the mode in Illustrator.
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Answers
Click on the Image Menu, Mode, then change it to whatever you want.
You mention Illustrator in the tags, maybe you saved an image from illustrator? If Illustrator is set to CMYK, it will output CMYK files whenever possible, and opening these files in photoshop will put photoshop in that mode as well. Use document color more in the file menu to change the mode in Illustrator.