My French Horn is playing notes wrong.?

Question:for instance when i try to play a C it comes out as a B. I left in in the trunk for a day. Is that why it's playing wrong. If it is please tell me how to fix it! Thank you.

Best Answer - Chosen by Asker: for instance when i try to play a C it comes out as a B. I left in in the trunk for a day. Is that why it's playing wrong. If it is please tell me how to fix it! Thank you.

Any change of pitch due to leaving it in a to cold or warm invironment would only be temporary and no big deal. Assuming that wasn't it check the following:
1. valve slides in the correct location and not pulled to far
2. check valve strings to make sure you don't have a broken one or that one is stretched to far.
3. anything stuck in bell or lead pipe
4. check the calibration on your tuner, set to 440hz

hmmm well if you let it get really, cold, it will be really flat, so i wouldn't be surprised if it was a whole half a step lower. I'd pull out you're main tuning slide quite a bit and check again with a tuner. Also

Thy to avoid putting you're french horn in the trunk its really bad for the instrument

As the daughter AND mother of two really great horn players, I'd say that you should take your horn to someone good and reliable who knows about horn repair (maybe a local music store where they sell horns might have someone who does this. There may be a simple explanation, but I would get it checked out.
Enjoy your music!

It sounds as if you put some tubing in wrong. When you take out your tubing to dump spit, did you put them back in the correct spot? Putting them back incorrectly can change the direction of air flow.

The french horn is one of the instruments that can change pitch without using the keys. You can do an entire major scale without using your fingers. You may want to try adjusting your mouth position, since that can modify pitch. If that does not correct the problem, you may want to go to a local music store and have an expert look at it.

I don't really think leaving it in a trunk for a day, or even longer, would change the pitch, and even if it did, once it warmed up, that would go away. I've never even heard of a horn changing pitch that much from a mechanical issue, but I suppose anything is possible. Have you checked it to a tuner? That may let you know more. You may wish to have a good teacher look at it too, before you consider a repair shop.