PHP Bug Fix Required

I operate a telecoms company, we had a PHP website created by a freelancer who is no longer available to contact and we currently have a problem uploading a CSV file for pricing, when we use the 'browse' and then 'upload' buttons and click 'go' the website directs to the '[login to view URL]' page however the following message occurs: "An unexpected system error occured. The error has been reported to the site administrator. You might want to hit the back button and try again. Sorry about this inconvenience." This used to work, I have uploaded two files ([login to view URL]) where we browse for the file and click 'upload' and [login to view URL] which is the page where we see the error. Thanks Gareth