Hi Saurabha,
First See below screenshot.
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Now, Its time to solve your problem.
If you use "layout tab", it let you design layout and form even in Display Mode, but you willl not get a "save button" to save your work, that's why when you activate this you work disappeared.
Always use "Layout Button", It opens a new window to design layout and form. At this window you will get a save button to save you work, after saving click close button, it will take you back, now activate your form. Now your hard work would not be disappeared.
Please reward if it helps you.
Regards
Piyush