Emergency! How do I debug this?

[Brian Dunning]

My web site has always worked, I didn't change anything, so whatever  
is causing this error happened all by itself:

#1017 - Can't find file: './kessler/products.frm' (errno: 13)

In phpMyAdmin, it shows the tables are "in use" and there's no way to  
execute a repair or anything. I don't know what else to try. Can  
someone please suggest a next step?