Check the affected space
Note whether the problem happens in one channel, a direct message or every conversation. A read-only channel or a restriction on new direct messages can prevent sending while the message history remains available.
Collect the details
Save an unsent draft, reload the page once and check whether the same issue affects another conversation. Confirm the channel is not read-only and read any restriction banner. Record the exact error, time and timezone, browser and conversation URL. Check whether other pages load normally.
Ask for help
Use the Messaging option under Technical issue or bug. Do not repeatedly press Send after an error; check whether your message arrived before trying again. A support agent can review your report without your password.