Unable to load workflow action from server. please contact your server administrtation

It could be an incorrect entry in the web.config file (sharepoint's), the .actions file for the new action..... look for the dll in your GAC and get the Public Key Token, compare that to the .actions file and the web.config file

which dll i should look for in GAC . Sharepoint workflow worked fine after install send email extended activity. But now i get this error

Unable to load workflow action from server. please contact your server administrtation

I had this issue a while back shortly after a custom webpart was deployed... I would only get the message sporadically, sometimes they would load and sometimes they wouldn't.  You may need to reboot your server and do an iisreset  (you can try just the iisreset first).
(I asked you to look in the GAC since I thought this was directly after the workflow action install, but if it did work right after then this probably isn't the case)

its bean a while i installed the send email extended activity & in between i think we got the situation to reboot the server. What is the best way to fix this

We need to find the root of the cause to find out how to fix it.  When you're opening SharePoint Designer are you opening on the server that hosts SharePoint or on your workstation?  If you're opening it on your workstation, try opening it from the SharePoint server.  If it works there then it's a connection issue (timeout, etc.)

It's a very generic error that can be caused by one (or many) of multiple different things.  We cannot find the cause without going through troubleshooting motions.

it worked after reintsalling the send email extended feature