Drupal site not responding anymore and warning messages

I installed drupal 6 and some additional modules (admin menu, cfk editor and IMCE). Because I installed the additional modules in the core module folder, I disabled and deinstalled them, moved them to the sites/all folder and installed them again. I only created some 3 or 4 text only nodes. Shortly after that, the site didn't respond anymore (after logging in). A couple of ours later, the site responded again, but everything I do causes the following two warning messages:

"user warning: Can't open file: 'r5yi_watchdog.MYI' (errno: 145) query: INSERT INTO r5yi_watchdog (uid, type, message, variables, severity, link, location, referer, hostname, timestamp) VALUES (1, 'user', 'Session opened for %name.', 'a:1:{s:5:\"%name\";s:5:\"admin\";}', 5, '', 'http://www.nvwc.nl/drupal/node?destination=node', 'http://www.nvwc.nl/drupal/', '83.163.177.95', 1259521850) in /home/danvwc/domains/nvwc.nl/public_html/drupal/modules/dblog/dblog.module on line 144."

and

"Warning: Can't open file: 'r5yi_watchdog.MYI' (errno: 145) query: INSERT INTO r5yi_watchdog (uid, type, message, variables, severity, link, location, referer, hostname, timestamp) VALUES (1, 'php', '%message in %file on line %line.', 'a:4:{s:6:\"%error\";s:12:\"user warning\";s:8:\"%message\";s:523:\"Can't open file: 'r5yi_watchdog.MYI' (errno: 145)\nquery: INSERT INTO r5yi_watchdog\n (uid, type, message, variables, severity, link, location, referer, hostname, timestamp)\n VALUES\n (1, 'locale', 'Parsed JavaScript file %file.', 'a:1:{s:5:\\"%file\\";s:14:\\"misc/drupal.js\\";}', 5, '', 'http://www.nvwc.nl/drupal/admin/build/modules', 'http://www.nvwc.nl/drupal/admin/build/modules', '83.163.177.95', 1259355374)\";s:5:\"%file\&quo in /home/danvwc/domains/nvwc.nl/public_html/drupal/includes/database.mysql.inc on line 128"

Url of the site is www.nvwc.nl/drupal