I have computer which acts as dhcp server. I want to ask that if i restart that server then will the other user be able to access internet while that is restarting
The other systems should be fine during the restart, unless their DHCP leases expire. A system which has a lease expire will be down until it gets a new IP, either through DHCP or through manual settings. Depending on how long your leases are set, this may or may not be a likely occurrence.
Note that this is assuming that your DHCP box is just that, and not also a gateway or something else.
Yes, the Internet will still work for users who already have a valid DHCP lease (thus an IP address).
BTW, Users who don't already have a DHCP Lease (thus no IP address), will have to wait for the server to finish starting before getting on the Internet.
The DHCP lease length will play part into this since your client computers won't normally try to renew their lease until half way into the lease's time length. For this reason I set my DHCP leases to 7 days or 24 hours, depending on the network design.