If your Drupal 6.x site is going offline frequently in a regular interval of time then your Backup & Migrate module might be causing that problem. What seems to happen is when the module initiates the backup at the predefined time period the site goes into offline mode [that's if you've configured it that way] but is unable to come back to the online mode.
If the culprit is Backup & Migrate then follow the instruction below to fix it:
1. Go to /admin/content/backup_migrate/profile
2. If you've created a profile other than the Default Settings profile edit that profile else edit the Default Setting profile.
3. Click on Advanced Options and Uncheck the box "Take site offline."
That should help.
If the culprit is Backup & Migrate then follow the instruction below to fix it:
1. Go to /admin/content/backup_migrate/profile
2. If you've created a profile other than the Default Settings profile edit that profile else edit the Default Setting profile.
3. Click on Advanced Options and Uncheck the box "Take site offline."
That should help.