At first glance, I don't see anything obviously wrong with your code, though I cant tell from your snippet how you have assigned the value of $email, $subject, or $content.
When you say it's "not working", what are you seeing happening? Are there any error messages? Any bounces or anything that indicates anything at all has happened? Does the mail() function return value of "true" or "false"?
I'm thinking the value returned by mail() is your first step, as it will tell you whether or not the mail was successfully accepted for delivery.
Also, depending upon what you mean by "not working", the mail might just be delayed for some server/queue related reason (which might make sense as the code "worked" before) - that's why I suggest determining value the mail functions returns. From the PHP manual:
If the mail was accepted for delivery, that helps in trying to figure out what else might be happening.