It sounds like you would probably want to use the Class Statistics tool, which can be found in the ENVI Toolbox under Classification > Post Classification > Class Statistics. This tool will take as input your classification result, and the input image, and will calculate a variety of stats for each class. I'm not sure what could be causing your data to disappear. Can you provide more information about exactly what you are seeing? For example, do you see the classification results in the Layer Manager after the classification finishes, and then you don't see them, without doing anything to remove them? Where are you expecting to find data that you do not see it? - Peg Exelis VIS

I already tried, but Class Statistic doesn't work. Let me explain my issue a little better. Before I apply the classification my image shows data for each pixel (temperature in K) and I can check that with Cursor Location/Value Window. However once I apply the classification the Data in the Cursor Location/Value Window shows only the number of my class (for example I have 4 classes: water, vegetation, concrete, and unknown) and the numbers go through 1 to 4 so when I move the cursor across one class all I see is that one number instead of different data for each pixel. Therefore Class Statistic doesn't work as well and in my case the idea is to get mean temperature for each class separately if that makes sense. Thanks again.

What you describe is normal for the output from a classification in ENVI. You should end up with one display layer containing this color-coded image with the same value for every pixel assigned to a given class. The original input data should still be available in another display layer (if it was displayed before you ran the classification). Then you use both of those images together in the Class Statistics tool. The color-coded image is the one you choose as the Classification Input File. The original image is the one you choose as the Statistics Input File. Then ENVI can calculate the statistics for each class, based on pixel values in the original image. - Peg Exelis VIS

Ah, sorry, I see that you are using ENVI Classic. My discussion of display layers only applies to the regular ENVI interface, where you can have multiple display layers in the same display. But the principles of how the Class Statistics tool works are the same. After your classification you will have an output file, which is color-coded as you describe. But you should still also have the original image data. If both of these images are open in ENVI, then you should see them both in the Available Bands List, and you can display either or both of them in their own display. When using the Class Statistics tool, you will choose the image output from the classification tool as the Classification Input File, and the original image as the Statistics Input File. - Peg Exelis VIS