Wiring codes may have a thing or two to say about fixing wires with electrical tape and burying that part in plaster. This may be noncompliant and possibly illegal where you live.
You’re lucky that the damaged wire is in a conduit—it should be possible to pull in a new wire, which is the safest way of fixing this.
Disconnect power before you do anything.
Now find out where the damaged wire begins and ends—likely at the next switch, wall outlet or junction box.
Then, determine the gauge of the wire and be sure to get a replacement wire of equal gauge.
Now, disconnect both ends of the wire. Attach the new wire to one end of the old one (e.g. by stripping a bit of the outer sheath of the new cable, forming hooks from the leads and securing them with tape—you don’t want the junction to come loose or get caught). Then start pulling gently at the other end of the wire, while another person feeds the new wire into the conduit.
When you have the old wire out and the new one in, cut, strip insulation and reconnect. Double-check all connections before you power up again.
One last word: if you really feel you needed instructions as detailed as these, consider getting a professional electrician to do this operation for you. Remember electricity can kill you or set your house on fire if you don’t know what you’re doing.