It can be a number of things, and you are going to have to open up the dryer to check things out.

First take off the back service panel and inspect the belt. See if it is tight around the motor and the tension pulley. If that is ok, then you need to open up the dryer cabinet.

You do this by inserting a screw driver or putty knife in the gap between the top and the front near the edges to release clips that hold the top to the cabinet. When the clips are released the top will flip up.

You can now take out the screws that hold the front panel on and disconnect the door switch, marking the wires to put them back correctly, and take it off. Release the belt from the back and left out the drum.

You should now be able to find your problem. If everything is ok and the drum turns freely your motor may be failing.