couldn't access to it "Not Found The requested document was not found on this server."
(1) First of all, please check that the 'admin/index.php' file is present on your web server. It sounds like the file might not be there.
(2) Next, check the permissions of the 'admin/index.php' file (and the permissions of the 'admin' folder and all other parent folders). Default permissions of 644 for files and 755 for folders should be fine. You should be able to check and change file and folder permissions using an FTP program such as Filezilla or via your web hosting account's online control panel.
(3) Try visiting your admin section by going to 'admin/index.php' (stating the index page's filename) instead of just 'admin/' in case your web server is not set up to find and go to the 'index.php' file automatically.
but i didnt changed something .
(1) If things were working fine and you did not change anything yourself but things are no longer working as they should, then this suggests that something may have changed on your web server.
Please ask your web host if they have made any changes to your hosting account recently which might somehow be contributing to your problem. Maybe they have changed some PHP settings or the PHP version.
(2) Also, please try installing and running the Showkase Server Compatibility Test.
Even if you ran the test before initially installing Showkase, please try again in case your web host has made any changes recently and any of the tests currently fail.
The results of the tests may point us in the right direction.
(3) Try upgrading to the current version of Showkase (v1.7.3) to see if this helps.
I have sent you a new download link for your Showkase purchase.
For reference, instructions for Upgrading Showkase can be found here.
Hopefully my notes above will help to resolve your problem (or at least point us in the right direction).
Please let me know how you get on and if I can be of any further assistance.