My horn is not working at all on my 2005 prius??? What to do??

There is a 10 amp horn fuse either in your cabin fuse box or the engine compartment fuse box. If it is not the fuse, you will need to troubleshoot to find the cause. First, find the horn relay (usually in the engine compartment relay box) and pull it out. Have someone hold the horn button down while you push the relay back into it's socket. The relay should click as you push it in. If it clicks, your problem is probably in the wiring to the horns. You'll have to trace the wires from the horns to the relay box to see if the connector went bad or what happened. If the relay doesn't click, you can try replacing the relay, but the problem may also be with the horn button. To troubleshoot the button, you need a 12 volt test light. You can get one for a few bucks at a parts store or WALMART auto tools isle.
Use the test light to interrogate the relay socket. Remove the relay from it's socket. connect the clip of your test light to the positive side of your battery and then probe all of the relay socket terminals while someone holds the horn button down. One of the terminals should turn the light on bright. Some of the terminals may light it dimly, but at most one will make it bright. If none makes it bright, your horn button is not working or the wire from the horn button to the relay box is cut. If you have questions, please let me know. If you want to understand the relay, see my post at http://www.fixya.com/cars/r6022358-relay_check