TL;DR: select Show Addresses item from the View menu. Then click the Addresses tab.

First, I had a bitcoin address ...

A Hierarchical Deterministic (HD) wallet has many addresses.

... and I used it once to receive BTCs and then it changed.

That is normal, for privacy reasons, an HD wallet will generate a new receive address for each transaction in which you will receive BTCs.

I can't find my bitcoin address

Look under the "Addresses" tab

(from a beginners guide)

It will be hidden by default. To reveal it, select Show Addresses item from the View menu.

i checked according awebanalysis.com/en/bitcoin-address-validate

I don't know that site but maybe you mis-typed an address or that site doesn't know about the type of address your wallet is using. Some online sites are not up-to-date with the latest Bitcoin features (e.g. Bech32 type addresses that start bc1...). Try another, for example http://lenschulwitz.com/base58