I replace my spark plug yesterday but today my car won't start what can i do

Definatley got the leads on the wrong way as mentioned earlier.
always mark the leads before removing, most newer cars you may be able to guess by the length of the lead, just go the shortest to the closest spark plug,, but will not always work a 6 cyl normally is 1-5-3-6-2-4 one being the closest to the front of the engine

If your car was starting up before you replaced the spark plug leads, my suggestion (as long as you put simlar plugs back in) is that the leads are around the wrong way........with the engine, if you have a 4 cyl, it does not fire 1-2-3-4, there is a particular firing order, eg 1-3-4-2, for a 4 cyl.

you did not state what vehicle you had, so i cannot tell you what the firing order is, and hence the correct position for the leads

this would be my first guess