Assuming that you are not right up against the deadline, the first port of call would be IT support, who would try to help with your submission, or would pass you through to the SALT team if they could not solve the issue.
If you are on Campus you can find the IT support desk in the main catalogue hall of the library, alongside the Issue Desk. This is the preferable option as staff will be able to see what is happening when the fault occurs, making it easier to troubleshoot. However, if you are not able to get to the desk, you can phone them on ext. 5060 or 295060 from an outside line.
If you are worried because you have left it until the last minute to submit and you think you may now not make the deadline, make sure you contact either the lecturer responsible for the assignment and or your college office. They should be able to advise you on what steps your college would like you to take in these circumstances. For example, they may suggest that you turn in a hard copy at the office, or e-mail a digital copy to them as evidence that you were there in time even if you failed to submit through the system. Processes for such occurrences may vary from college to college, so do make sure you seek advice from those concerned, rather than just from a friend who had previously had been in this situation.