i sent a message as a text and not it wont send as a imessage for just that person.

imessage wouldnt send so sent it as a text. now it won't imessage that person anymore. how can i make it imessage again?

First, try rebooting your device. If that doesn't fix the issue, assume that the recipient has disabled iMessage or is not connected to the Internet via Wi-Fi or cellular data.