If you are a relatively new user, eBay may limit the number of messages you can send in a day. This is to prevent someone from registering a new account specifically to send spam messages or offers. Such limits typically reset after 24 hours, so you may be able to try again later.
If you were initially sending messages through an item listing, and if the item has ended or expired, you may no longer be able to send or reply to messages related to that item at some point.
Or possibly the seller has added you to the seller's blocked bidder list -- perhaps on purpose, or perhaps by mistake. If you think the seller has made such a mistake, you can try contacting the seller through the seller's feedback or profile page using the Contact link there.
