My phone can receive texts just fine, but whenever I want to reply back to a received text, it won't send at all: "Message Send Failed. Message will be saved in your Outbox." something like that.

But my reception was perfectly fine, all 5 bars.
This is the first time it's done that... Do you know what's going on?

try taking the battery out, counting to 30 and then putting it back in..that acts like a manual re-set and then try to send a text message and see what it says..its wierd that would happen if you have perfect service...if it continues take it into a repair center and have them check it out.