i don t understand, i was open ticket 7 days ago , no one anser me.
i have really problem what’s up people.
up to os 4.0 everything working normal, but when i try hier (4.1-5.7) versions dhcp server after reboot not working
Master Reset and reconfigure 
are you kidding ?
I would do a reset of the system and reconfigure.
The DHCP server does work in those versions, but given how the RouterOS configuration language tends to change, you might have something from an older version that it doesn’t like.
I wouldn’t go to the full reset path, but probably would drop the old DHCP config and redo that bit. Just fire up webfig (which is hugely more advanced in 5.x than in 4.x) or winbox and redo the configuration under /ip dhcp-server print detail, /ip pool print detail, …
Also make sure you have the right packages installed (/system package print) and check your firewall filters to make sure nothing is stopping the traffic.
If you export the relevant bits of your configuration and post them we might be able to make much more specific recommendations.