My 30 yr. old Curtis Mathis console tv screen went black tonight after a white line went across the center of the black screen. Won't come back on. Is the main tube blown?

Any time you have just a white or a colored horizontal line across the "Middle" of your screen it means you have a loss of vertical deflection. This can be the result of:bad connections, a bad vertical output driver I.C., & usually its going to be bad capacitors. Other less common problems are a loss of voltage to driver circuit and on rare occasion a bad yoke.
When the picture collapses to a vertical line down the center of your screen, it means a loss of horizontal drive from the horizontal output stages which include the horiz. oscillator, horiz. output transistor, the flyback transformer & its associated capacitors, diodes, and voltage dropping resistors in the lower voltage and higher voltage screen circuits. A less common problem for this would be an open in the yoke circuit.
In this old of a set, bad solder joints in any of the above mentioned circuits should be a first consideration especially around the flyback secondary windings and the voltage input points in the oscillator stages. I have found everyone of these problems over the many years I serviced these older sets.

Unplug it,then wait 10 minutes,then plug it back in.