Backups! You do have backups? Contact your hosting support if not sure and ask them. Clean the site completely and restore from backup pre dating the compromise, also first change all site/server access, that is FTP passwords and also set up new DB user passwords and when back up running change all WP user passwords.
I really ought to wait for you to post first 
should have known there were guides 
Otto’s and Donncha’s posts are excellent ones on the general subject of compromised sites.
I think, given the OP generic style question, that the first port of call is to your hosting support, and if they are a good support team and hosting company that they will advise on how you should proceed as there are many variables in this equation and many take some detailed knowledge.
However the rough steps – and important first things you must do and ensure – that you should be taking initially, before tackling the actual site restore are:
1/ Ensure you are no longer serving pages publicly -in other words make sure the site is no longer running.
2/ Change all access codes to the site FTP, Admin Control Panels, SSH (if you have that form of access)
When these steps are completed you can begin to think about cleaning the site and restoring from safe backups, but do take any advise and help your hosting support are willing to give.
Oh and sweep your personal computers for root kits, trojans, key loggers and don’t sweep once sweep multiple times with two or three apps.
In addition to what @hnla said above, if you cannot log in your WP installation
https://codex.wordpress.org/Resetting_Your_Password#Through_phpMyAdmin
I’m going to install this tonight: https://wordpress.org/extend/plugins/wordpress-firewall-2/
Wordpress firewall. I would consider it on your install as well.
@gunju2221 so how’s the plugin working out or you?
hang the firewall I’m wondering where the OP has got to
Maybe to the WP.org forums …
Yep probably, must have got it back up and running I guess, just glad we could help
