Seized brakes are a bummer. Usually it happens due to rust arresting parts which should be moveable.
Drum brakes are easier. You will notice the cable connecting to a lever which comes out of the face of the drum. Give it a couple whacks with a hammer away from the cable, and see if that frees it up. If it does, disassemble, lubricate everything. Replace the drum shoes as needed, and adjust it properly.
Disc brakes can be a little more challenging. 1. Take off the brake caliper from the bracket, while keeping the brake line connected. Note whether or not the brake pads were rusted to the rotor, that might have been the problem. If not, then support the caliper so it does not pull on the brake line. Remove the brake pads. Remove the cap to the master cylinder up at the handlebars. 2. Spray them down with some WD40 or whatever penetrating oil you have. Using a large pair of channel locks or a c-clamp, gently compress the caliper piston.
Notice that you may have 1 or 2 pistons. Gently compress them in until they seat. Now spray everything down again with oil. Now, slowly squeeze the brake handle. They should start to come out again. Don't do it too much or they will fall out altogether. Just send them out about an inch. 3. Squeeze them in again with your clamp. Then, out again with the brake handle. Do this a couple more times. Now, wipe everything down with a rag, and blast all the WD40 away with your air compressor. Do it a couple more times, looking closely for brake fluid leaks. If you find some, the calipers will need to be rebuilt or replaced. 4. Now reassemble everything, and bleed the brakes.
If it is your master cylinder which is seized, you will notice that the little plunger which sits outside of the master cylinder, and gets pushed by the lever, will not pop back when you release the lever. Personally I take this as a sign that it should be replaced, but some people rebuild them with kits found online.