What version of MATLAB are you using, what GPU and what platform?
This sounds like a problem seen before with workstation 'Sleep' or 'Suspend' power modes. Some period after you sign out your workstation enables this mode and GPU compute stops functioning in running applications. Typically this can be avoided by disabling Sleep mode, closing MATLAB before signing out, or ensuring that you are driving your display with a different GPU than you are using for compute.
Some versions of this issue have fixes, depending on your card and OS. You can also try updating your GPU driver.