Dashboard not accessable

Hi,
My dashboard suddenly stopped working and showing me this error

Fatal error: Class ‘WP_Styles’ not found in /home/f9np6cwi/public_html/wp-includes/script-loader.php on line 858

I don’t know what happened, the last thing i did was installed the ‘nextgen gallery plugin’, and as soon as i activated it, the page refreshed and this error came up, since then i cannot access the admin panel.
The website is working fine, for public i have published a coming soon page. I think the problem is with wordpress files, because i can see the admin website front-end but when i try to access wordpress dashboard it shows this error. Help wanted, will be waiting for you reply.

Regards,
Yasir

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