Power cut out on tascam x9

Since nothing else is working, the cooling fan is probably powered by AC at line voltage.

The power supply that feeds the electronics has failed but it may not be catastrophic; it might just be an internal fuse that has given up from metal fatigue.
Each time current flows when powered up, the delicate element expands and then contracts when the power is turned off.
Some fuses have a zig-zag crimp in the element that helps delay the eventual death.

With the unit unplugged, remove whatever screws you find on the back or bottom of the mixer so you can peer inside.
If you are lucky, you may find the fuse in a clip or otherwise plugged in.
There will definitely be one in the incoming AC line although if the fan is running, this one may be OK.
There could be one (or more) fuses on the DC side that directly services the circuitry.
Fuses come in a variety of cases; they can be cylindrical of glass or ceramic bodies with metal caps on the ends and of varying lengths.
Others may be vertically mounted smaller cylinders plugged into or soldered (bad) to the board.
Some are nearly indistinguishable in size and mounting from several other types of parts; these though, will not have other than simple text on the part while the others will have more complex markings.
If you find and replace fuses, buy two so that an eventual tech doesn't have to grub around to find one.

If you replace any fuses and they fail again, DON'T proceed. You may damage the unit further and not help anything.

At that point, you need to turn it over to a tech.