Probably cause "Plain Text" has been checked.
If your Classic account has been updated----there will be these links near the composing page----"Rich Text or Plain Text" that enables or disables that bar.
Click Rich Text to enable it.
If Plain Text has been clicked---the bar is removed.
In the original version---checking the button--"compose with color and graphics" ---should enable it.
You DID remember to click "save" at the bottom of the page----didn't you ????