I am using electrum. First, I had a bitcoin address and I used it once to receive BTCs and then it changed. Will I be able to receive Bitcoins now, even if someone sends them to the old address?
Yes, you will still receive Bitcoin if someone sends to your old addresses.
Electrum uses mnemonic seeds to generate private keys which has corresponding public addresses (keys) - the ones you need to provide someone to send Bitcoin over to you. This means every transaction to addresses which are generated by your mnemonic seeds will still go to your Electrum wallet even when your receiving address is now changed.
Just in case you wonder why the receiving address keeps changing time to time: the ideas of changing public address every time you receive some Bitcoins is to keep your balance info private as with a public address since one can look up and list all transactions related to it using Bitcoin explorers such as blockchain.info.
Yes, you are still able to receive money sent to that address. Your Electrum wallet doesn't forget the private keys to addresses it displayed to you in the past and you can receive arbitrarily many payments to the same address. It just discourages using a single address to receive money more than once.
Receiving money several times to the same address causes privacy problems because people (or their computers) may be able to gain information about the payments someone receives and may even be able to identify you using additional information.
Electrum will always provide new addresses if you used the old ones. On a desktop computer (you can use the same Electrum wallet on different devices), you can see the used addresses by switching to the tab
Addresses and expanding the