[Mediawiki-l] Sudden failure on missing .MYI files

[Michelle]

Our wiki failed suddenly two days ago with a complaint that it can't  
find user_groups.MYI:

Query: SELECT ug_group FROM `user_groups` WHERE ug_user = '3'
Function: User::loadFromDatabase
Error: 1017 Can't find file: 'user_groups.MYI' (errno: 2) (localhost)

Also I can no longer look at the dbase in phpMyAdmin because  
"page.MYI" is not found.

The thing is, looking at a backup of the dbase when things were  
working fine, there was no user_groups.MYI or page.MYI then either.   
There are a bunch of 'tables' that have only .frm files: page,  
pagelinks, revision, validate, user_groups.  I honestly have no idea  
what a .frm file does by itself.  Are these obsolete files leftover  
from upgrades?  But then, why is the code looking for them?

My ISP has no idea what to do, apparently.  Does anyone have any  
hints?  I have a backup of the dbase but I'm not sure what to do or  
what to tell the ISP to do.  This is 2 years of work and we're  
freaking :(

Install info:
MW 1.5.8
MySQL 4.0.18
Apache/Linux hosting (iPower)

Thanks so much,
Michelle