when I look at your profile, I see 5 people on your friends list. if you only had 5 friends, then its most likely just a communication issue with the server.

If you had more then the 5 friends mentioned above, it's most likely your account is compromised and someone has access to it.

Do the following to secure your account before they do any more damage to your account:

1. Scan for malware https://www.malwarebytes.com/
2. Check that the email and phone number on the Steam account are still yours.
3. Deauthorize all other devices https://psteamproxy.yuanyoumao.com/twofactor/manage
4. Change passwords from a trusted/clean device.
5. Generate new backup codes for your Mobile App https://psteamproxy.yuanyoumao.com/twofactor/manage
6. Revoke the API key https://psteamcommunity.yuanyoumao.com/dev/apikey (there should be nothing in the APIKEY)

引用自 Wolf Knight：

when I look at your profile, I see 5 people on your friends list. if you only had 5 friends, then its most likely just a communication issue with the server.

Actually, I only accepted these people recently, after realizing that my entire list had disappeared; I had almost 200 friends and they all vanished.

引用自 miamew3：

If you had more then the 5 friends mentioned above, it's most likely your account is compromised and someone has access to it.

Do the following to secure your account before they do any more damage to your account:

1. Scan for malware https://www.malwarebytes.com/
2. Check that the email and phone number on the Steam account are still yours.
3. Deauthorize all other devices https://psteamproxy.yuanyoumao.com/twofactor/manage
4. Change passwords from a trusted/clean device.
5. Generate new backup codes for your Mobile App https://psteamproxy.yuanyoumao.com/twofactor/manage
6. Revoke the API key https://psteamcommunity.yuanyoumao.com/dev/apikey (there should be nothing in the APIKEY)

There were indeed almost 200 friends. Thank you for your attention and advice. I will follow all the steps, but is there any way to recover the list without having to add everyone again?

If the number of community and/or trade banned people in your comments is any indication, you were indeed hijacked and should probably re-evaluate what you're doing online.

For friends, try...

Navigate to... Steam\userdata\<Steam3ID>\config\

Open localconfig.vdf up with WordPad and CTRL+F to your friends. Find your friends.

https://psteamcommunity.yuanyoumao.com/discussions/forum/1/492378806385644508/#c492378806386027594

Some sites cache previous friends like Steamid.uk and Steamrep.