Having an issue starting a login session

Users and me at my website are having issues when they try to log in.
When ever they log in and try to start a session, it just brings them back to the homepage as if they never logged in at all.
This problem did not occur until earlier today, and I had not altered any of the files in the ftp.

I am just wondering what could be the causes of this issue, I will post the code from the login php file and so forth and would be grateful if anyone could point out any potential issues.

if($_POST[‘action’] == “process”) {
userlogin($_POST[‘username’], $_POST[‘password’]);

if(!isset($username) | !isset($password)) {


Username: Password:

Forget your username/password? <? } ?>[/php]