My Drupal 6 website has been working fine for many months.

Now all of a sudden, it's displaying the following error messages:

    Warning: Can't find record in 'cache' query: SELECT data, created, headers, expire, serialized FROM cache WHERE cid = 'variables' in /home/olo000/public_html/<<mysite>>/includes/database.mysql.inc on line 135
    
    Warning: Cannot modify header information - headers already sent by (output started at /home/olo000/public_html/<<mysite>>/includes/database.mysql.inc:135) in /home/olo000/public_html/<<mysite>>/includes/bootstrap.inc on line 729
    
    Warning: Cannot modify header information - headers already sent by (output started at /home/olo000/public_html/<<mysite>>/includes/database.mysql.inc:135) in /home/olo000/public_html/<<mysite>>/includes/bootstrap.inc on line 730
    
    Warning: Cannot modify header information - headers already sent by (output started at /home/olo000/public_html/<<mysite>>/includes/database.mysql.inc:135) in /home/olo000/public_html/<<mysite>>/includes/bootstrap.inc on line 731
    
    Warning: Cannot modify header information - headers already sent by (output started at /home/olo000/public_html/<<mysite>>/includes/database.mysql.inc:135) in /home/olo000/public_html/<<mysite>>/includes/bootstrap.inc on line 732

How would I go about solving this?

Steps I have tried:
I've searched extensively for other threads that discuss the same problem, but I've not found any.