Do alts vanish when not used for a while?

[Rolig Loon 27,286]

Nope. Accounts are forever.  Basic ones anyway.  If you have a Premium acount and haven't kept up your membership fees, LL will cancel it, but not a Basic account.  My guess is that you were looking for a Display Name, which is unsearchable.  You have to look for the login name.  Try looking in the AppData area on your hard drive to see if you saved any chat or IM messages related to that alt.  They'd be in the SecondLife>>> Roaming folder.

[Rolig Loon 27,286]

Nope. Accounts are forever.  Basic ones anyway.  If you have a Premium acount and haven't kept up your membership fees, LL will cancel it, but not a Basic account.  My guess is that you were looking for a Display Name, which is unsearchable.  You have to look for the login name.  Try looking in the AppData area on your hard drive to see if you saved any chat or IM messages related to that alt.  They'd be in the SecondLife>>> Roaming folder.

[Bixyl Shuftan 9]

I never gave the account a separate display name.

[Rolig Loon 27,286]

The only other possibility I can think of is that you were banned, so Linden Lab removed your avatar permanently. Otherwise, if you are sure that you are entering the correct name, it should show up in search.