my dell optiplex 320 desktop freezes booting

dell optiplex 320 desktop stops when the dell logo appears. is not able to continue to POST.i read on the net that i can pull off the power cable and then press on the power knob for ten seconds.I did that and it worked but after restarting the computer it happened again.When it happens so the USB ports don't function. Can some one help me solve it. Thank you.

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