There are a few reasons your brakes may not be making contact with your tire.
First, double check that your brake cables are connected, this happens more often then you would think.
If you have traditional brakes your cables may just need to be adjusted (tightened if your brakes aren't touching the rim when you squeeze the handle), or, you might need new pads if they are worn out.
If you have hydraulic disk brakes you most likely need to add additional brake fluid.
You should always give your brakes a little test first thing when you jump on your bike!