- how are you sending the mail -- using the mail() function?
- what is the return status of the mail() function (TRUE or FALSE)?
- what is in the web server's error_log?
- what changed in the web server config?
- what changed in the PHP config?
- what changed in the sending mail server?
- has your form been abused by spammers and the mail server admin decided to no longer relay your mails?
- what changed elsewhere in the sending system?
- what changed in the recipient's mail server?
- are new anti-spam rules in effect in the receiving server?
- is the sending mail server blacklisted on a DNSBL that is used by the receiving system for filtering?
- did you sent the mail with a valid return address to see a bounce?
- what is the bounce message then, if there is any?
- what changed at the receiving mail client?
- are new anti-spam rules in effect at the client?
- has the mail been mislaid at the client's end (perhaps it is in the spam folder, if there is any).
This list may be incomplete, though.
But as long as you don't give another hint on what part may "not work", it is hard to tell anything ....