Errors such as: ..."Got error: 1016: Can't open file: 'YOUR_TABLE_NAME.MYI'. (error:145) when using LOCK TABLES"... (as well as numerous other noted below) are caused by MySQL table corruption. Table corruption can be caused by a variety of events, and will make some or all of a table unavailable.
Table corruption can also significan'tly decrease the performance of queries on effected tables. Table corruption can be be repaired by using the the REPAIR TABLE command.
For more information about the REPAIR TABLE command see:
MySQL 4.0, 4.1
http://dev.mysql.com/doc/refman/4.1/en/repair-table.html
MySQL5.0
http://dev.mysql.com/doc/refman/5.0/en/repair-table.html
If phpMyAdmin is unable to display the table information, or encounters errors loading the table details page, another option would be to use the SQL button at the top of the left frame to enter your own query. You can repair the table manually by issuing the command REPAIR TABLE <tablename> in this SQL window.
This command can take some time and will block some queries, such as inserts, to the table.
Some other errors that may be caused by table corruption include: