Hello, Without the information I asked for in my request, I can only give you this general answer but I hope this is helpful.
If you are unable to send email from your iPad, it is possible that your iPad has lost connection with the email server. So you can remove the account and add it back and this will make the iPad to re-establish the email connection back.
Please follow the steps below
Step 1: Open the Settings App from your Home screen.
Step 2: Select the Mail option from the list. If using old iOS, then you may see Accounts and Passwords OR Mail, Contacts, Calendar
Step 3: Select Accounts
Step 4: Select your email account
Step 5: Select remove account. Skip to the next step if you don't see an account to delete and then add it
(Please keep in mind this will delete the emails from the iPad but the emails will be re-downloaded back when you add the account back if they are available on the server. Also, you must add it back with the correct password)
Step 6: Select Add Account and select your email account to add.
Make sure you add it with the correct password. It will add your email account to your iPad and you can send and receive it now.
Please try the above-mentioned steps and reply back to me if you still need assistance.