Issue Raised : user not able to access or use some keyboard keys, like Backspace, CTRL, Delete in Oracle SQL Developer.
Open SQL Developer and click on Tools menu > select Preferences > Accelerators > click on Load Preset button on the right hand side > select Default > click OK twice to close the preference window.
Now it should work fine with the All keyboard keys.
Important Note : In Updated SQL Developer "Accelerator" Menu is replaced with "Shortcut Keys"
Open SQL Developer > Tools > Preferences > Shortcut Keys > More Actions > Load Keyboard Scheme and select "Default" > click on "OK" and save.