Book of Face end-to-end encryption HELP

Okay people can anyone help me I use face book mostly to keep in touch with friends via messenger now I get this message and cannot read their replies or indeed see the message I sent. This is the what I get a computer generated
Messenger upgraded the security of this chat. New messages and calls are secured with end-to-end encryption

I can send message and obviously they can read them as they are replying thing is I cannot see the messages?

try clicking on restore now and get the pin number sent to your mobile phone
Enter your PIN to restore your chat history

(Some messages are missing. Enter your PIN to restore your chat history.)

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Okay Thank you where do I find restore now?

at the bottom of one of your chats click the restore then you can select forgot pin and i think it then will say enter mobile or email
restore

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Thank you for your help complete idiot here,

Right I get this NO restore now

Messenger upgraded the security of this chat. New messages and calls are secured with end-to-end encryption.Learn more

should say messages are missing then click the restore now next you get the pop up

like this

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No not getting messages missing or anything just the as above
Messenger upgraded the security of this chat. New messages and calls are secured with end-to-end encryption.Learn more

No idea how to restore the messages

I can see people have seen the message but cannot read anything I send to them or they send to me. Totally baffled