Most tests are completed on a "first come, first served" basis. When an email notification comes into your inbox, it's likely being seen by many other contributors at the same time. By the time you log in, that test may have already been started by another contributor.
If the email indicated the test was specifically reserved for you by a customer and you don't see it, contact the Support Team.
I received an email about a new test. Why can't I find it on my dashboard?
