How can I bootup my Mac Mini?

Dear Sir,

I didn't use my Mac mini for 2 years. But when I tried to reboot it last week, it seemed that it always hold on the status bar went to 75%, and then stopped. I had tried a lot of time to reboot it. It kept the same result. As if the hard driver went out, would there be any possible I can get the USB disk to bootup the system, and try to fix the problem?

Thanks!