I have a Sears "Professional" 15" variable speed lathe.

Model 351.217.156

I've had it for about 4 years, but to be honest I've used my small
Jet Mini lathe a lot more.

I've turned maybe 5 bowls, a few wands and other spindle turning on it.

So, today I put on another wand, turn the power switch on, and it's
going at full tilt (2000 rpm). So, while it's running, it start turning
it down to 400 (it's slow speed), partway down the knob feels loose, and
when I remove the cover, I see that the belt is loose, and if I
turn it down to 400 it's so loose the spindle stops entirely, even
though the motor is still spinning at full speed.

How easily is this fixed? I knew that a sears lathe would not last
as long as others, but with light use I figured it would last a
LOT longer.

And advice would be appreciated,

Phil