I've got an old glass bong that has a crack running up the side. It's not broken, and it's still watertight, but that crack is still there, and I know it's only going to get worse.
Can it be heated up and "melted" back together? I'd just throw it away, but it has a lot of sentimental value (original owner has passed away).
it can be fixed, at a risk though.
borosilicate starts moving at around 1200 degrees. at 1500 degrees it is almost dripping. your piece needs to be "soaked" in a kiln for an hour somewhere around 1050 degrees. and then CAREFULLY brought into a flame. at this point it may or may not explode. a lot depends on the original working of the glass and the quality of it as well.
if it can be brought into the flame without exploding then it should be easily fixable. simply start beyond the crack with a nice penetrating flame and work your way back to it. by starting ahead of the crack you prevent if from traveling any further. it may leave some bubbles behind within the glass, but they won't hurt anything.
it can also be fixed inside the kiln if the person doing it happens to have a small hand torch. put the piece in the kiln for an hour at 1050/1100ish, then open the kiln door and work right inside it. this method will reduce the risk of exploding by quite a bit. it's the first initial "shock" of changing temps that causing it to explode. thew closer the glass is to working temp when you start the better.
try googling "glass blowing" in your area for local artists who may help you. costs can vary drastically.