Typical issues with electric knives include bent blades, inability to turn on and a noisy motor, so realigning the blades, checking for loose connections and cleaning the assembly and contacts are all useful initial steps. Replacing worn out parts is also a possible solution.
If the electric knife is slicing poorly, the blade might be bent. Visually inspect the blade for any curvature. Carefully press the bent place against a flat surface. If the bend persists, the blade needs to be replaced.
For a noisy or non-functioning motor, the casing needs to be opened. Unplug the knife, and remove the blade. Use a screwdriver to remove the screws holding the casing together. Carefully pull the casing apart, watching for the blade retainers to drop out. Unscrew the driver assembly and motor from the housing. If the motor is non-functioning the problem is probably either loose wires or the switch. Inspect the motor for loose wires, tightening them as necessary. Spray electrical contact cleaner onto the switch to clean it.
A noisy motor is probably caused by worn drive gears. In that case, disassemble the motor to expose the gears. Remove each gear, and inspect it for wear or damage. Replace them as necessary, and reassemble the motor.