$wpdb seems to have stopped working

I have these lines of code in a custom plugin:

$table = $table_prefix.”posts”;
$sql = “select * from $table where post_status = ‘publish’ and post_type = ‘post'”;
$undone = $wpdb->get_results($sql);

Up until a few days ago, this piece of code had worked fine. After some testing, I know this is where the code is failing. The entire function just stops working after this line now.

There were some updates/upgrades done to my server and around this time, this line in the code stopped working. I am not sure if it was the changes to the server or something else.

The file has not been touched in well over a year and has always worked fine prior to this. I am baffled as to why it stopped working so any suggestions on where to look to fix this would be appreciated.