Host updated PHP version without telling me

My current hosting provider migrated servers and the new server had php5. Now a particular blog will not load. and this error appears.

Warning: require_once(/home/leoniew/public_html/blog/wp-load.php)
[function.require-once]: failed to open stream: No such file or
directory in /home/leoniew/public_html/blog/wp-blog-header.php on line 12

Fatal error: require_once() [function.require]: Failed opening required
‘/home/leoniew/public_html/blog/wp-load.php’
(include_path=’.:/usr/lib/php:/usr/local/lib/php’) in
/home/leoniew/public_html/blog/wp-blog-header.php on line 12

The hosts comment re the migration was “you will have to get your developer to fix it, a few of our clients have had this issue” arrghh i wish they notified us clients before they did it.

Im sure there are a lot of users here who would be able to offer some help, im very appreciative.

Thanks in advance
Amy