In this case, either I need to change/fix my theme’s code. But, what if I am not a coder?

The solution is to change the currently active theme from database.

Open phpMyAdmin, go to your wp database. Open wp_options table. Search for three option_name – template, stylesheet, and current_theme.

SELECT *
FROM wp_options
WHERE option_name = 'template'
OR option_name = 'stylesheet'
OR option_name = 'current_theme';

See the option_value fields from the search result. The option_value will be your theme name. Change option_value of the search result to ‘classic’ OR ‘default’. Classic and default are the default wordpress themes present at the time of wp installation.

UPDATE wp_options SET option_value = 'default' WHERE option_name = 'template';
 
UPDATE wp_options SET option_value = 'default' WHERE option_name = 'stylesheet';
 
UPDATE wp_options SET option_value = 'default' WHERE option_name = 'current_theme';

In this case, you need to consider few things to make the site working. You need to make changes in wp-config.php (present in root directory of wordpress installation) and some update in wp-options table of your database.

Changes in wp-config.php

I suppose that my database name is wordpress, database username is root, password is pass and database server name is localhost.

/** The name of the database for WordPress */
define('DB_NAME', 'wordpress');
 
/** MySQL database username */
define('DB_USER', 'root');
 
/** MySQL database password */
define('DB_PASSWORD', 'pass');
 
/** MySQL hostname */
define('DB_HOST', 'localhost');

Changes in database (wp-options table)

1) In wp-options table, search for siteurl and home in option_name column.

SELECT * FROM `wp_options` where `option_name` = 'siteurl' OR `option_name` = 'home';

2) Then edit the option_value column for the two rows with your current site url. I have supposed your current url as http://example.com. You can put your own url.

UPDATE `wp_options` SET `option_value` = 'http://example.com' WHERE `wp_options`.`option_name` = 'home' ;
 
UPDATE `wp_options` SET `option_value` = 'http://example.com' WHERE `wp_options`.`option_name` = 'siteurl' ;