PDO Exception Error - How to resolve this?
Could be a MySQL configuration issue - see http://dev.mysql.com/doc/refman/5.5/en/too-many-connections.html

Or, quoting after http://drupal.org/node/930876#comment-4519490:


  If MySQL connections are not closed
  properly, then you might have an error
  within the code somewhere, that leads
  the script to end prematurely.
  
  In standard cases, connections are
  properly closed.
  
  Look at the detailed error log of your
  server, or activate on page error
  reporting if you're on a develpment
  site. You might find something useful,
  such as a forgotten dsm() somewhere
  with the devel module not being
  activated.
  
  The error might not be displayed and
  leads the site to look as usual, but
  the PHP process is not ended properly,
  leaving MySQL connections opened.


Start with checking your error logs and try to find out why this could be happening there.