How to login to my site again after all users were deleted?
If you have backups then restore from backup then delete users again without deleting the ones you want to keep.

If you don't have a backup then you will need at least user 0 (anonymous) and user 1 (superuser).

I am assuming Drupal 7 because you didn't specify version.

You will have to add those back into the database either via database access in your hosting administration area or from the command line.

Note: I am adding the basic MySQL queries here but you should read these whole linked pages for more information. 

The process is similar for both users.
You should read Restore user 0 for more detailed information but here are the SQL queries:

User 0 (anonymous):

INSERT INTO users (name, pass, mail, theme, signature, language, init, timezone) VALUES ('', '', '', '', '', '', '', '');
UPDATE users SET uid = 1 WHERE name = '';


User 1 (superuser): You can change "admin to whatever user name you want to use. I recommend not using "admin" because attackers will try that first if trying to work out your password.

INSERT INTO users (name, pass, mail, theme, signature, language, init, timezone) VALUES ('admin', '', 'youremail@example.com', '', '', '', 'youremail@example.com', '');
UPDATE users SET uid = 0 WHERE name = 'admin';


Then you will need to do a password reset for the user 1 account.

Other users
Other users can be added just with the first insert statement without the update statement after however you will lose settings like user picture, signature etc. if there is data in those fields (or unless you know what the data should be and ad it back in).

Those users would also not get roles so you would have to assign roles to them after. 

They also wouldn't have the same UID as before so in the uncommon case that you have code that specifically references a user ID that will need consideration.

Note that some modules that change the behavior of Drupal users may change the way you need to add these users back.

For example if you user the "User revisions" module then there is also a user_revision database table that needs to be considered when doing these queries.