When you are 'missing' someone they are absent and you regret their loss and you wish so much to see and meet them again.So those persons are out of reach at the moment you miss them. It involves feelings of sadness of not being with them anymore. " His wife died last year, he still misses her very much." " Will he miss her if she didn't contact him ?" 'To think' of someone or something is to keep them or it in your mind for attention or consideration. You could think of someone who is present and you could reach them easily.It doesn't necessarily involve any emotional longing, so it has a different meaning from 'miss' and is not interchangeable with it. " He thinks highly of this person". However 'To think about' someone is to have them on your mind and think actively about them. " Since he met her, he thinks about her day and night." Here there is an active thinking about some present person ,that one could reach and meet easily. When you 'miss' someone you are also thinking about them actively,but this is then associated with a feeling of sadness because of their absence and a longing to see them,that can't be fuliflled presently.

think of 只是说明“在想”、“正好想到”、“考虑到”，而 miss 的意思是指“想念”，突出已经好久没有见面了，程度要比 think of 要强烈和正式，带有个人感情色彩在里面，而 think of 用的很随便。