Sounds like a root problem to me.
a - overwatering leading to rotten roots
b - they are in a plastic pot which you hide by placing it in a decorative ceramic pot. Again, this will lead to rotten roots.
c - there is a soil bound pest attacking the roots.

Of course, I am assuming that you're growing this as a houseplant. It wouldn't have a hope in hell outdoors.

So how can I check if it realy is the rotten roots, make sure that it does not rott again and make it grow again?
It is in quite small plastic pot, but not hidden in decorative pot.

Just pull the pot off and see if it is wet or slimy. They do loose leaves, sometimes quite a lot. If healthy then they do grow back quite quickly if cut. Perhaps a larger pot and sun light would help.

Which Dracaena is it? If you don't know, a photograph would be very helpful, preferably one showing the whole thing, including the pot. If its in a small plastic pot, it sounds as if it needs potting on into a larger one. Dracaenas of any variety do not appreciate sunlight, but most like good daylight, although there are one or two which tolerate low light conditions quite well.

I think it's Dracaena Marginata, I added a photo of it :
www.growsonyou.com/kincha

I took it out of pot and checked the roots, they seem to be fine(nice and yellowish), there are quite big roots on the bottom of the pot with no soil, like if it needs deeper pot...could that cause the leave drop?

I'm going to pot it in larger pot.Any suggestions what to definitely do when potting?which pot is better, which soil?
and what to do with its top?shall I cut the leaves?cut the stem?just leave it like this?

Thanks a lot to everyone!

I've had a look at your pictures. It looks to me as if the plant has been left waterlogged at some point - is that possible? There appears to be rotting at the tips. The pot is also a good size already.
I think emergency resuscitation is required, so here's what I'd do. Cut down the tops to about one inch from the main stem with a sharp, clean pair of secateurs, provided the short lengths left on top are not mushy as well. If they are mushy, you will have to cut them all off by removing the top half inch of the main stem. Repot in fresh compost - if the roots are coiled at the bottom, and the whole rootball is completely solid with roots, you will need a slightly larger pot. If, though, there are coiled roots at the bottom, but plenty of soil within the root ball, then just carefully remove any loose compost that you can (it should be very damp to do that), then put back in the same pot with new compost, ensuring you don't break the roots, and also ensuring you don't leave any air pockets, and the roots are fully surrounded with compost.
The tuft of green growth on the right hand side in the picture - if the growth itself is still healthy, remove the woody part at the base, but not so far that the leaves fall apart- stick it in a bottle of water on a windowsill, and wait. In about 6-8 weeks, it will have formed roots, and you can then pot it up.
Subsequent care - remove the pebbles from the tray beneath. Water well after you've finished repotting, leave any water which runs out in the tray for 30 minutes. Then empty the tray, and empty again 30 minutes later if there is any more water sitting there.
Future watering - only water when the surface of the compost is dry to the touch, probably about once a week. When you do water, water well, and empty the tray 30 minutes later. Never leave the plant standing in water.
After 6 weeks, feed during the growing season (March through to about August) once a week or fortnight with Baby Bio houseplant food. Keep away from heat sources like radiators, and direct sunlight.
This may seem rather drastic - but you need to take drastic action. Provided the rot is not in the mainstem or roots, the plant will recover well and grow on.

Thanks.