Click on a blank part of the task bar. Keep the mouse key depressed and drag the bar back to the bottom of the screen. (In my experience, it usually takes several attempts until you find the knack).
To stop it happening again, right-click on the task bar. Select 'Properties' and click the 'Advanced' tab. Under 'Task bar', click to remove the tick alongside 'Enable moving & resizing'. Click 'Apply', then 'OK'.
Right click on the Task Bar and make sure there is no tick next to 'Lock the Taskbar'.
Then left click in a blank area of the Task Bar and hold the left mouse button down. Drag the Task Bar to the bottom. Let go of the left mouse button. Lock the Task Bar again.
Have you tried pressing the windows button (between the Ctrl and Alt keys), or Ctrl+Esc keys?
Also try moving the cursor to the bottom of the screen (do it with the sides as well if the bottom doesn't work).
See if the cursor changes to a small double ended arrow. If it does click and hold the left mouse button and drag upwards. You may have minimised the task bar.