can't send or receive from my iPad after accidentally updating to 14.3

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID, to send SMS/MMS messages from your iPad you must explicitly verify that Text Message Forwarding is enabled for your iPad on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

First thing what you can’t send or receive

Second thing update your iPad to iPadOS 14.4.1 this one is the latest one

I can’t make out what you are meaning to ask but you can try restarting your iPad learn how to restart it

https://support.apple.com/en-us/HT210631

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

Did you downgrade or upgrade if you downgraded probably update your iPad to latest one that is iPadOS 14.4.1 and wait for 48 hours if problem persists contact Apple support link