Iconia A1-811 refuses to boot nor to hard reset

DeepPurple

I have an Iconia A1-811 for month. It worked without incident until one week ago. Began to freeze, then to make wild reboot. I have tried a soft reset. But now, it only shows the Acer welcome logo and then I get a blank screen. I have tried the hard reset procedure (with volume up and power press). Without any result. How could I force a hard reset of it ?