The Metrics page is a really useful tool for seeing what's been going on with the resource usage on your server. It can give you an insight in to what was going on with the processors, disk and memory on the server over the last hour, 24 hours and 2 weeks.
From the server dashboard select the Metrics page
We've given each of the resources different colours so you can differentiate the resources to make it easier to read and broken down the time intervals to every couple of minutes (on the 1 hour view) so you can pinpoint the time of something easier. From there you can take a look in the sites logs to see get a bit more detail about what was going on, on the site at that time.
We've got some tips and hints for troubleshooting high CPU in a guide here.