in bitcoin you can reuse an address as many times as you want but it is not recommended. instead you should only use each address once (receive one payment in it, and receive others in another address). to do that wallets such as Electrum change the receiving address as soon as the previous one received payment. that is why the address you see in your Receive tab is different now.
however your wallet still has that address and its key and it can be reused and if you had received any coins in that address they are spendable.
you can click on View from the menu and select Show addresses to see a new tab that contains all your addresses.
also your payment (if you have actually received it) should show up in your History tab with the date and amount and if you double click it you can see more details including the Transaction ID which you should have received from the sender in your chat.