Theme page is not showing

May be you installed some other theme version which is not capable your core version og druapl.
For Eg Your drupal core bersion is 6.6 and Theme must be applicable for your core version.
The above issues apply for modules also.

may be your last uploaded theme will cause your problem, Just remove and check.

"Page doesn't exists" means your modules folder may be deleted check it in your server.

There may have been an error in the theme or you may have hit your max. memory limit during the installation step. Both of these can corrupt the databases menu structure.

Have you still got access to the menu pages? Editing things here will rebuild the menu router table.

If not, fingers crossed, it only effects the cached tables; try to flush the cache (in the site config > performance), otherwise manually truncate the cache tables if you do not have access to these. It may pay to disable the theme, this can be done by setting the status flag in the systems table to zero.

In case you not sure about these step, take a database dump first.

It this does not work, running update.php may help. From memory it rebuilds the menu router table. I've never had to take things this far.

Otherwise it may come down to a backup.