Can a nameserver end with a . (dot) like:
ns100.hostname.com.

or sould just be
ns100.hostname.com

Is this for a zone file, if so then yes you need the period at the end.

Well I have registered a domain through Namecheap and have my website at another host. I need to give the right nameserver to namecheap.

Then you shouldn't need to add the period at the end, if this is with Namecheap.

Okay, I might just be patient then.
It's been about 26 hours since i change the nameserver but it does not yet work.
I get "server was not found or DNS error"

Agree with Pat H, you don't need a dot at the end.
DNS propagation can take up to 48hrs(but usually takes much faster) so wait a while more.

Have you ran your domain through DNSreport lately?

http://dnsreport.com

It will show which DNS it has a the root servers, in the zone files; as well as most of the common problems.

Good luck!

your ISP might cache them for more days (its usually 24-72 for big ISPs to save bandwidth)

your ISP might cache them for more days (its usually 24-72 for big ISPs to save bandwidth)

Bashar, you don't really mean 24-72 *DAYS*, do you??? :)

It should be 24-72 *HOURS*.

lol my bad forget to add hrs after the numbers :D