Issue with wp-db.php (whole site down)

Hi all

I have been working on a site for a fair while. I just installed bbPress and added a forum (no problem) but then, when I added a second forum, the whole site fell over.

Now, whenever I try to get to my page, I get this message:

Warning: mysql_set_charset(): Error executing query in /srv/disk5/1830043/www/WEBSITEADDRESS/wp-includes/wp-db.php on line 723

Warning: Cannot modify header information – headers already sent by (output started at /srv/disk5/1830043/www/WEBSITEADDRESS/wp-includes/wp-db.php:723) in /srv/disk5/1830043/www/WEBSITEADDRESS/wp-includes/pluggable.php on line 1178

I have no idea what to change. I can log in to my hosting account, but I have no idea what to change to fix this. I gather it’s something to do with wp-db.php, but if I don’t know what I’m doing, I’m certainly not going to randomly change something in wp-db.php.

Please help – I really need to fix this ASAP ??