Hiya
This is a mysql table problem, possibly corrupted; I've had 2 such problems in the 2 weeks
Try going into the mysql command prompt; choose the moodle database and type
use moodle;
the repair the table with this line
repair table mdl_cache_text;
Maybe this will work from phpmyadmin/moodle database admin as an sql query, to save going into the mysql prompt
Cheers, Andy D