My sister is depressed, how can I help!

My youngest sister is depressed, and I don't know how to help her. She used to be funny and love hanging out with friends, but now she doesn’t have any friends. It seems like she can't keep up with people and doesn’t reply to any texts on WhatsApp, which upsets her old friends or at least they don’t text her anymore. She doesn’t leave her bed and has become too lazy to study or do the activities she used to enjoy. She’s also become very shy and insecure. I’ve heard that she was diagnosed with masqued depression

Bref, I need your perspective on how I can help her Ps : she is 20 and I'm 25