Oh dear You can't send your draft email because the buton "send" is greyed out and you cant save it either because "cancel" is also greyed out?! Yikes!. Fortunately here is the solution.
It's going to be scary, but trust me, this works.:
Here's my fix (which I can vouch for) skip down a bit to try an easier method.
While you have your draft email still open (with the non responsive buttons), press and hold the home button and the sleep wake button for a few seconds. Most of you will know by now that doing this allows you to completely turn off the iPad. The screen changes and the "slide to power off" screen appears.
Although it will be quite scary (because you will think you will loose the unsaved draft email), slide and turn off the iPad. Not to worry, this is an iPad not a regular computer just turn it on again, go back to the mail app and you will see you can now send the email or save it as a draft.
EDIT: EASIER METHOD:
Robert commented below, that you don't need to turn the iPad off. His fix is this:
- Bring up the list of running apps by either double pressing the home button or you can three finger swipe up.
- Hold you finger on the mail app and when it starts wiggling, tap the red cirle with the minus symbol to close the mail app.
- Restart the mail app and you should get your mail back so you can send it or save it as a draft.
NOTE: I give no warranty that this will work perfectly. I have also not yet tried roberts method. If you do actually lose the email don't come crying to me, I'm only trying to help! (I dont really think you'll lose the email, it's an iPad after all!).