What causes this error?

[Goanna]

I launched a site for someone and it was working perfectly, and then all of a sudden it went down today and displayed an error with whos_online.MYI

 

I went to phpMyAdmin and clicked on it and got this

Can't open file: 'whos_online.MYI'. (errno: 145) 

Error

SQL-query :  

SHOW KEYS FROM `whos_online` 

MySQL said: 


Can't open file: 'whos_online.MYI'. (errno: 145)

 

I was able to fix it simply by dropping the table and copying a new one over. I am just curious as to what caused this? Could it have been to many people loggin on at once? The site has been up for 3 days and already has over 100 customers and like 6000 hits or something. It said specifically that the error hapened from user session ********* and to delete it. But I couldnt access the file so I droped it and copied the new one over.