There will probably be four wires going to the timer. These will be: Live and Neural, which run the timer, plus two other wires. One feeds Live from the oven switch and the other goes to the oven itself.
It is the second two wires that will get connected together by a relay in the timer.
Identify that second pair of wires and connect them together.
Alternatively, since you'll have to take the timer out anyway, get a new one and fit it properly.
Jude make sure you make a note of which wire goes where