So firstly and most importantly, what version of Exchange and OWA are you using? 2003 or 2007?
This is important as the mechanisms for testing and debugging are different.

Normally when OWA just suddenly "stops" working for everyone is due to something like an upgrade
as previously noted on this forum, the Exchange 2007 SP2 Rollup 2 causes things to not work, but the icons should still show
but a lot of times if its Exchange 2003, you would normally see this if there is a hotfix or upgrade on the OWA side that updates the Control files

So questions are

1. What version of Exchange are you running?
2. If you open up Add/Remove programs, can you see whether any windows updates or hotfixes were applied recently?
3. If its exchange 2003, does it work if you go directly to the back end exchange server as opposed to the front end server?
4. You will need to enable logging really to determine more whats going on

If you are on Exchange 2007, follow the following technote to enable logging for OWA2007 and EV
http://seer.entsupport.symantec.com/docs/304386.htm

Also if its Exchange 2007, check out this article, you may need to read it through a few times as it isn't the clearest
http://seer.entsupport.symantec.com/docs/321015.htm

If its Exchange 2003, you will need to log on to the backend server, go to your EV installation directory where the EVBackend.ini is, which is usually
C:\Program Files\Enterprise Vault\OWA\EVBackend.ini

Open this up and change the following line so that =0 becomes =2
yourExchSvr.1.exchangediagnostics=0

Once done, log out and back in to your OWA, you will then have a file in your logging directory that will show whats going on.

General tips are you should troubleshoot directly to the backend or CAS Server
so for instance a lot of companies will have something similar to the following

https://owa.myCompany.com -> frontEnd1.myCompany.com
https://owa.myCompany.com -> frontEnd2.myCompany.com
https://owa.myCompany.com -> frontEnd3.myCompany.com

So if you go to owa.myCompany.com, it may be impossible or very difficult to determine which front end you are genuinely are going to
so cut out the owa.mycompany.com redirector and go to the front end or back end URL's directly
If the URL is Network Load Balanced, then you will have to go to the IP Address directly as opposed to the host name.

this way you may find that it is just one of x amount of servers that aren't working correctly.

And with Exchange 2003, always work on the backend servers first to get them working before you concentrate on the frontends

 We are running Exchange 2003, 
We had only one Hotfix  Kb943545 (STOR PORT Hotfix) installed recently 

i not able to see the icons even if my logon to the back end Server 

I enabled exchangediagnostics=2 on my back end servers  and logged on to my OWA both from Front end and backend servers but no logs gets created on the logging directory  

OK so the question is, when you log in to OWA on the backend, do you see the icons at least for archived items?
i.e as opposed to the Store in vault, restore buttons and archive explore buttons, if you find an archived item, does it show the Archived icon?
Also if you open the user in Outlook with the EV Outlook add in installed, can you see the icons there as well?

So heres the first thing we need to do....

1. Determine what version of the control files you're using
    - Log on to the Backend Exchange Server
    - Go to Start -> Run -> Type "logfiles" and press Enter
    - Go to W3SVC1 directory and open the latest log file
    - Look for a line such as the following
      2009-06-01 15:16:13 W3SVC1 192.66.66.1 GET /exchweb/6.5.7638.1/controls/style30.css 
    - Note the numbers next to /exchweb/ as this is the Control file version (6.5.7638.1)

2. Make sure the EV files are installed there
    - Go to the backend exchange machine
    - In Windows Explorer browse to C:\Program Files\exchsrvr\exchweb\[ControlFiles version from the previous step]
    - Ensure there is a folder named "controls-originals copied by Enterprise Vault"
    - Also look at the other controls files in the \exchweb\ directory and see if EV has been installed there

If the control files were modified under a different control set, re-run the installation so that it modifies the correct control file versions.

Other common causes for things like this can be
 - A new mailbox store is registed....EV configuration runs against each mailbox store against the SystemMailbox and each time a new mailbox store is added to exchange, the configuration has to be re-run so that it can detect the new stores and register the forms against them
 - Control file mismatch between front end and back end server
   EV will modify the control files of the latest version of the control files, so if you have say .1, .2 and .60 on the front end, it will only modify .60
   But if you only have .1 on the backend server, when you open up exchange , it will detect the latest control files on the backend server, see that its running .1 of the control files and run the .1 control files on the front end also, so therefor the icons will fail to show

Typically though if its a straight forward environment, sometimes its just easier to uninstall/reinstall and run the configuration, making sure obviously you have the EVServers.txt populated correctly, or setting the proxycfg -d