If it totally goes when you dis-engage the clutch, its most likely the gearbox main-shaft or counter-shaft end bearings if it sounds like that.
To be honest, if its suddenly gone and gone that bad, i would suspect there is an underlying problem such as an oil leak etc..
Unlikely to be the thrust bearing, that will be quiet when the clutch is not depressed and worse when it is depressed, due to it being under pressure in contact with the diaphragm springs, common misconception, or people refusing to believe their gearbox is nackerd
I would advise you to change or at least check the gearbox oil, remove the upper filler plug and fill it until it pours out, if it took more than half a litre its almost certainly lost oil causing the issue. If the level is ok, Run the engine in netueral and rev it, listen down close to the gearbox for rumbling/clicking noises, that will pretty much confirm it.
Could possibly be a faulty clutch plate falling apart, but very unlikely.
This is based on the info you have given so far so could be other things too but more info would be needed to go down any other route at this stage.
