The NoClassDefFoundError suggests the SQL Developer zip got corrupted / the unzip did not work properly. Best to download again -- 4.1.3 is the current production version -- and unzip into an empty directory. You may also wish to rename or delete your user settings under C:\Users\<your_user>\AppData\Roaming\SQL Developer\ system<sqldev-version> to make sure you are getting a completely clean install.
Thank you very much Gary Graham. No fresh installation was needed. Just renaming the SQL Developer folder(s) in C:\Users\<your_user>\AppData\Roaming\ and then re-starting the application did the job
It is like re-initializing the application.
Bibhuti Bhusan Padhi