I have run into this exactly same issue on three of my client websites in just the past 5 days. All three sites were version 2.5.X.
- open the database in question in phpMyAdmin.
- At the "home" screen, click on "databases" tab at the top.
- On the next screen, click on the name of the database.
- The list of tables and their structure will load into phpMyAdmin's main frame. Scroll down until you see the sessions table, and check it.
- Scroll the rest of the way down the page, and look for the "With Selected" pulldown menu at the bottom.
- Click the pulldown open and select "Repair table."
- phpMyAdmin will automatically process the request, repairing the table. When this is finished, you will see the following message at the top of the page:
- Your are finished, and the website should load just fine.
I have no idea of what might have caused three such crashes within days of one another on the same server, but when looked at my server logs, there was no indication of anything untoward going on. I spoke to my own php/mysql guru, he said that it was possible that someone was trying an sql injection or something, but without any server log indications, it would seem hard to tell what is going on. Everything on my server regarding linux, apache, php, and mysql is all up to date current versions.
In any case, you learn as you go forward. At least the fix was a simple one, and only takes about 30 seconds to do.