Most likely a stone amongst the coffee beans has become trapped between the two burrs.
Unplug the grinder from the electricity supply.
Turn the grinder upside down to empty all the coffee beans in the hopper into a tray
See if you can spot the stone jammed between the top and bottom burrs
Set the grinder to the coarsest setting available
(i.e. move the burr heads as far apart a possible)
Turn the grinder upside down again and see if you can tip the stone out, as it should have come loose.
If this doesn't work refer to the instructions that came with your grinder.
It may need to be returned to the retailer as stones can be difficult to remove from some grinders due to their design.
While we try our best to remove stones from our coffee, some still make it through.
We do apologise if you are ever inconvenienced by a stone from our coffee jamming in you grinder.
If you turn your grinder off as soon as you hear an unusual noise you will usually be able to remove the stone before it jams between the burrs.