Crashed tables in your database can happen for a number of reasons, for example issues your site or more commonly when the MySQL process is killed by the servers operating system whilst a table is open. This can lead to the table being marked as crashed and is therefore unable to be edited or exported.
A quick way to repair a crashed table is to run a MySQL check and repair on the database which will check the consistency of your database and fix any crashed tables. This can be performed on your sites database by clicking the repair button on the database for your site as shown below:
For more information about mysqlcheck please visit the following link: