If you can, try testing the drive on another computer.
If it can be recognised on that other system, then it is an issue with the USB/Firewire interface on your current system. That could be :
- a USB/firewire driver issue
- USB/firewire port problem
- some setting in your system's BIOS is disable your USB/firewire ports
If it cannot be recognised on that other system, it might be :
- the USB/firewire cable doesn't work properly
- USB/firewire port on your drive has dirty contacts
- something has gone wrong with the drive
As for why it suddenly, happened, I have no idea.