Magento 2 How to unlock reindex process

You can reset the indexer through command line with indexer:reset command.

This will give the list of indexes name:

php bin/magento indexer:info

Output:

design_config_grid                       Design Config Grid
customer_grid                            Customer Grid
catalog_category_product                 Category Products
catalog_product_category                 Product Categories
catalog_product_price                    Product Price
catalog_product_attribute                Product EAV
catalogsearch_fulltext                   Catalog Search
cataloginventory_stock                   Stock
catalogrule_rule                         Catalog Rule Product
catalogrule_product                      Catalog Product Rule

This will give the list of indexes status:

php bin/magento indexer:status

Output:

Design Config Grid:                                Ready
Customer Grid:                                     Ready
Category Products:                                 Ready
Product Categories:                                Ready
Product Price:                                     Ready
Product EAV:                                       Ready
Catalog Search:                                    Ready
Stock:                                             Processing
Catalog Rule Product:                              Ready
Catalog Product Rule:                              Ready

If you want to reset all the indexes, you can run the following command:

php bin/magento indexer:reset

If you want to reset particular index (e.g. cataloginventory_stock), you can run the following command:

php bin/magento indexer:reset cataloginventory_stock

When I faced this kind of situation I had to run the following SQL query directly into the database:

UPDATE indexer_state SET status = 'valid';

I was not able to to find any option to force reindexing when an index has failed previously.

On MySQL run:

SET SQL_SAFE_UPDATES = 0;
update indexer_state set status = 'invalid' where status != 'valid';

Then on your terminal run:

php bin/magento indexer:reindex

It normally happens when the memory limit is few, so increase on your .htaccess or NGINX config.