If the bar has only disappeared from your browser, pressing F11 is likely to bring it back. If not, right-click on a blank space at the top of the screen and click to add the missing toolbar.
If it's actually your task bar (at the foot of your screen, even when yo've not got any browser, or other program, open) which has disapppeared, move your cursor to the foot of the scren and it should change to a double-headed arrow. Click-and-drag upwards to where the top of the task bar should be.