I have mixer tascam tm-d4000 stopped working now shows on the screen "fatal error" what I have to do
I'd suggest flashing the onboard CMOS and check for updates. A fatal error is usually caused by corrupted programming on these units. If you're not comfortable flashing the CMOS I'd suggest taking this unit to a service and repair professional.
Flashing the CMOS is relatively easy. Take off the bottom cover and you'll see a ton of wires and a control board and the back end to the main computer unit. Open the secondary case up and look for a watch battery on board. Pop it out and let the unit sit for awhile (10 minutes to an hour). This is usually a good time to buy a replacement battery. Once you've got the new battery pop it back in and it should brought the unit back to a factory reset state. Any stored programs will have to be reprogrammed but the unit should work. If you still get a fatal error it might suggest hardware failure. Some of the newer models of the Tascam series mixers (including submodels of the TS4000 series) have a hardware diagnostic that is available, while some of the simpler units just give you their version of a blue screen of death. Which could indicate anything from a lose connection and an error reading a subcontrol, to a blown capacitor, blown fuse, short circuit. If it's still under warranty I'd speak to the manufacturer about getting it repaired. They might also offer more sufficient troubleshooting over the phone for your issue.
I do believe there is a factory reset button code you can also try for this unit, but in my experience this doesn't really clear the memory and reset the unit properly. It's like a warm reset instead of a proper reset.