How can I check why my website was down?

I would start by checking the Apache error log file for possible insight. Depending on how your web service is configured, you should be able to view the log file using one of the following commands:

less /var/log/apache2/error.log
less /var/log/httpd/error_log

I also recommend verifying that you aren't running into disk space and inode issues. If so, resizing your server's disk to a bigger size or resizing your Linode to a bigger sized plan should help with this matter. To verify the information, you can run the following commands:

df -h
df -t
Resize a Disk
Resize a Linode