Inability to send can sometimes occur if you've been in an area with poor coverage. The solution is usually to reset the device. There are two ways to do this. If you can resolve your problem by a soft reset, Alt-R.Shft-Del, you should. In this case, though, that may not work. If it doesn't, take out the battery with the phone on. Leave it out for about 30 seconds. Put it back. Among other things, that will force a rebuild of the indexes and reload the operating system into contiguous memory and may fix the problem.
When you pull the battery, there is a very slight risk of corrupting your OS, resulting in your having to wipe/reload your device. To be safe, be sure to perform a backup before the battery pull.
Jeff has a pretty good idea of what is going on, but I ran across a problem like this and some of my service book were corupted, to fix I just resent the service book from the BIS page and all was fine.
