So, I was able to find this info:
Friends.. All those who are facing the problem with pics disappearing in gallery , please note that the problem is with HTC modification in Android+HTC Sense combo. The same problem is found in HTC One V, X, S, Explorer, desire etc.
Whenever this happens, note that the pics in gallery have disappered showing "No photos or videos found" msg; check Music app also-You will NOT find the music files also. Although all the data is still on the SD card/phone memory it is not visible in Gallery/Music app.
The main problem is with "Search Index" of the phone. Whenever, you access your files using a third party file explorer & delete some data OR add some data by connecting usb to PC, the cache created by "search index" of HTC phones gets corrupted.
The only solution I could find is Go to>>Settings>>Apps>>All apps>>Gallery/Music>>clear cache & clear data. Then disable the app. Restart phone. Then again enable the app. Now when you check your gallery; all the files appear.
This bug can be solved by HTC technical team. For that we all need to contact them by sending Email/making calls.