1. Have you given your login details to someone else? The system only allows one person to be logged in at any one time with a single login profile, this means if you are logged in and then someone else logs in with the same login details, you will be logged out of the system. This is a security feature to make sure that your data is only accessible by people you know and trust
2. Have you been able to go to other websites? Sometimes, it is not the system that has logged you out but your internet connect which has dropped or had a "blip" which breaks your connection with your account. 
3. Has your account been inactive for more than 45 minutes? As a security feature, we have an automatic log out which kicks in around 45 minutes of inactivity, this helps to keep your data secure if you step away from your computer in a public location

If you are still being logged out, please raise a support ticket and we will take a look