WordPress crashes with “The service is unavailable.” after trying to upgrade plugins

This is definitely not error message produced by WordPress, so it comes from server software. Please check server logs (if you have access and skills) and ask your hosting support to look into it.

It is hard to make a good guess from this little information.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)