If you've been sending a lot of messages, there is a limit of 45 per day, and you should be able to send messages again within 24 hours. You may wish to exchange email addresses or phone numbers to avoid hitting the limit.
If you haven't been sending a lot of messages, or the problem lasts longer than 24 hours, then could you tell us how many messages you've sent, what device and browser you're using, and what happens when you try to send a message, including any error message, and if possible a screenshot?
One of these may help:
using this link: https://knowndonorregistry.com/mykdr/messages
clearing your browser cache
using PC/desktop mode if you're on a phone or tablet
using a different browser (you can run other browsers eg Chrome, Firefox & Opera on a phone or tablet)
using a different device
If you have any urgent messages, I could forward them for you, and pass on your email address if you'd like. You can also include an email address or phone number on your profile.