Likely causes:
Belt broken - easy to check -undo 3 screws and remove top of drier (unplug from mains). The Belt should be seen round the drum - its black and about 1" wide.
Timer stuck - try turning the timer control by hand.
Motor given up - perhaps needs new brushes or has burnt out.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix tumble dryers.