Mine has a trip switch on the back, its just a little red plastic dome (doesnt even look like a switch) but it cuts off the element when it overheats, maybe due to overloading and the heat being unable to escape quick enough - have a look if there is anything similar.
Ours was the same. Cold air only till I read the manual and pressed a small red switch at the back. The book called it a reset button. Worth looking at the back of your drier.
I would suggest either the element has gone, you could check that using a multi meter.Or the over-temperature stat has gone.On my tumbly this is a fusible link, i.e a component with solder inside which melts if the tumbly gets too hot.I replaced mine with one from R.S which was about 70p.