I have a simple question.

Can a virus be sequenced if it hasn't been isolated?

If yes, could you please tell me how it's done?

Depends on how you define isolated. But if you do not have its DNA in some form, you cannot sequence it. If you got a mi with something x, it is difficult but not necessarily impossible to identify the viral reads from the background. But unless you got a good reference to map the reads to, it would be very tricky indeed. 

^{Thanks Charon.}