Well, you've lost the data itself at that point in an image file format; you'll have to go back and regenerate the plot from the data that was used to call boxplot
IF you have another saved copy as a MATLAB .fig file, the data are in the file as it saves the content of the objects as well as just the image. How much is user visible in the boxplot object I don't recall OTOMH, probably the raw data are, but I wouldn't guarantee it with the penchant TMW has now for making these specialized plots far more opaque to the end user than needs be.
The short, easy answer is, however, to go back to the code from which the plots were created and save those data there. Hopefully you have the input files from whence it originally came at least.
