Welcome to BCF, and congrats on having an MG T series.
Funny coincidence - I've been working on my TD's speedometer, tachometer, and clock this week.
If your speedometer was working perfectly, then suddenly stopped (doesn't indicate speed), first thing I'd think is the pointer/needle might have got stuck on the rest stud. Shake the speedometer a bit and see if the needle moves at all. If so, then, check the cable connection at the back. Reach up through the access hole at the left of the steering column. The knurled ring might have worked loose, so the cable inside the sheath isn't turning the speedometer.
If that's tight - check that the cable is turning. Raise the rear wheels, put the car in gear, give it some gas, and see if the cable turns as the rear wheels turn.
Does the speedometer stay at zero *and* the odometer no longer registers mileage?
Let us know what you find.
Tom M.
1953 TD27318