I've been using 4XL for years without any problems. Today I had to load it on to a new laptop. At first all was fine and my projects were opened and played ok. Suddenly it stopped paying my projects. The wavs are there and can be seen but when pressing play no output occurs and nothing is played. 
Have I accidentally turned something off?
Anyone got any ideas

verify the timing masters are correct in preferences

Thanks for the reply. Had a look and there are no options for the Timing Master (only option is "none"). Also noticed that there is no output driver showing in the Drivers tab.
Any ideas?

If there is no timing master, there is no driver found. You need to select a driver mode supported by your audio interface. If it is using the internal sound chip, it is usually MME. Some use ASIO4All as a wrapper for the sound chip driver and select ASIO drver mode. It might provide better performance. If on the other hand you have a dedicated audio interface, SONAR does not see the drivers, the drivers are not installed or the wrong driver mode is selected. Dedicated audio interfaces usually use ASIO or WDM driver mode.

Hi, thanks for this. I selected MME and after a couple of re-starts it now works. Big thanks for your help on this