First checked the Drive motor and Drive roller placed correctly aligned.
Look at the the plastic pulley on the roller has not be shifted. could be it slipped and the knurling might not hold it securely. you could try tapping it back into alignment, but if it wont stay put then you'll need a new front roller.
Another options you may have This is typically caused by frame misalignment. If you fold up the treadmill, is the gap on each side by the handles the same? If not, the company from where you buy should be send you a replacement treadmill.