All your addresses can be found under the 'Address' tab.

Once a address has been used it is moved to a folder called 'Used' so if you do not see one of your older addresses then make sure to check the 'Used' folder.

And as others have already mentioned, all your addresses remain valid, they do not 'expire'. You can receive funds on any of the addresses so you can give a single address out multiple times for various transactions.

You mention you are new to 'Electrum', did you make a backup of the seed words? If not then please do and keep that backup in a safe place. Without that you will not be able to recover your Electrum wallet should something go wrong. Most of the problems posted here in regards to Electrum has to do with people who forgot to backup the seed words and because of this could not access and or recover their wallets.