出现这个情况的原因是对方删除你的时候QQ不会有任何提示，具体查看被删除好友及加回来方法如下（以windows10系统的QQ2019版为例）：
1、打开QQ，点击界面左下方的“三”按钮。
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2、在接着弹出的下拉菜单中点击界面右上方的消息管理按钮。
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3、在随后打开的界面中点击左下方的“已删除联系人”按钮。
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4、找到需要恢复的联系人，查看上方显示的QQ号码。
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5、在QQ界面中输入QQ号码，点击“+”号。
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6、随后等待对方通过验证或出现“已添加为好友”即可。
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