Synchronisation status
First check Multibit's synchronisation status.
It probably shows a message on the bottom left status bar "wallet synchronised with network" (perhaps depending on exact version of Multibit software)
If it isnt synchronised yet, that might explain why it doesn't show the balance you expected.
Recovery from wallet words
There are two main versions of Multibit: Classic and HD. Both are no longer supported.
To recover a wallet from your wallet words (also known as recovery phrase or seed phrase) follow the instructions at
https://multibit.org/help/hd0.1/restore-wallet.html
From the unlock screen click the "Restore" button and follow the instructions on the MultiBit HD screen.
There may be different instructions for different versions of Multibit (Classic, HD2, HD3 etc) - check the appropriate instructions for your exact version if possible.
It may take a while (days) to synchronise before it shows you a balance.
Multibit is unsupported
If your money becomes visible again, because Multibit wallet software is no longer supported, you should move your money into a different type of wallet. Multibit seem to suggest Electrum (electrum.org) - maybe that is closest to Multibit in terms of user interface? However there are many alternatives which you should probably consider.
Consider a hardware wallet or an offline-wallet/cold-wallet for longer term storage.
Did you ever write down your wallet words (seed phrase) or make a backup of the wallet? – RedGrittyBrick – 2019-07-18T07:47:08.223
I did write down the wallet words. I didn't make a backup of the wallet. – Carnabwth – 2019-07-18T17:19:51.163
I don's see anywhere to input those seed words. – Carnabwth – 2019-07-18T17:35:51.107