Site shows wordpress installation page

What is the URL?

accapastpapers.com.pk

That’s the install.php file. Just fill it up and complete installation.

But it tries to install and gives error of an existing wp folder. I don’t know what happened all of a sudden.

Is it a new site or do you have content in that that you don’t want to lose?

I had content and it two years old site.

Backup your site and try:
– Deactivate all plugins and see if it helps. If so, activate plugins one by one till you find the problematic one.
– Revert to Twenty Twelve theme to rule out any theme-specific issues
– Reinstall WordPress manually except wp-contents
Use FTP or your hosting control panel if you do not have access to your dashboard.

I have no access to the site. How am I supposed to deactivate the plugins?

Use FTP or your hosting control panel if you do not have access to your dashboard.