ご無沙汰しております (Gobusata shite orimasu) is a very polite and formal way to say "Long time no see." You use it when addressing superiors, business clients, or anyone you wish to show high respect to, acknowledging that you haven't been in touch for a while.

目上の人やお客様に使う、丁寧な「ご無沙汰しています」です。久しぶりに連絡を取る際に使います。