I have optiplex dell 620 its held on bious logo , i reset my bious , changed my ram and cpu , disconect hard and optical drive but it still held on bious logo & no any key of keyboard working ,
how can i fix it

Hold Down the Fn Button during startup to access the dell diagnostics. See if they run.

The D620 is an antique (but still usable) in internet terms. I had a D600, D620, and now a D630 (which is about 6-7 years old now). If you can't get to the diagnostics partition (as per CimmerianX's instructions), then you should contact Dell tech support. They do system repairs (probably replacing bad parts with remanufactured ones at this point) at a reasonable cost. The main thing is the hard drive. You need to check that first, because it has all your software and data. IE, if it is functional, don't send it in with the system as you will probably get a new one of those as well, and hence lose everything on the HD.