There is no information about this in my dictionary. Unfortunately, for me they have exactly the same meaning. Could you explain the difference to me if I am wrong?
沒想到 - something you should have thought of but didn't. [It is a surprise that...]
- 沒想到他不降反昇 - It is a surprise that he was not demoted but promoted instead.
想不到 - something that couldn't have thought of beforehand. [couldn't think of that...]
想不到他竟然昇官了 - Couldn't think of that he was actually promoted.
我想不到了 - (I am done with guesses), I couldn't think of any more.
In general, the construction "verb + 不 + verbal suffix" can be paraphased in this manner: "I did try to do, by did not manage to get the expected result". For example:
In these examples, the "expected result" (or intentional goal) is indicated by the lexical value of the verbal suffix (i.e. 干净, 完, 醒)。
In another thread, I have explained that the verbal suffix 到 conveys the meaning of "manage to do something" when the context is associated with an intentional goal. For example:
- 我找她很多次了，但找不到。(I tried to find, but did not manage to find.)
- 她去广东想吃荔枝，但荔枝季节已经过了，吃不到。(She tried to eat, but did not manage to eat.)
In this case, the intentional goal is not a particular value of the process, but just the process itself.
to me a native Ch speaker, they are just the same, if we were really to nitpick, 没想到 focus on the surprise whereas 想不到 on out of expectation. this one below may help put you in perspective at a 20yr reunion, if you say to someone:
没想到你当大官了 （it says nothing about his aptitude for a bureaucrat, but i'm just so surprised， equivalent in En would be: it has never occurred to me that you could be a big shot in gov) 想不到你当大官 （same suprise, but just a trace of "how could someone like you, who was so anti establishment, become a bureaucrat", the focus is on the contrast)
but, this is just nitpicking and maybe just me. basically, you can safely interchange these two in daily conversation. P.S. 想不到你竟然当大官了 fits better 不揣陋见 仅供参考
They're similar in that they're both used to express surprise at something unexpected, and in this case you're not really meant to interpret it as related to failing to think (想) in some way:
我没想到这个小餐馆的菜这么好吃。 (I didn't think this small restaurant's food would be so delicious.)
我没想到这件事情会变得这么复杂。 (I didn't expect this issue to become so complicated.)
这个新闻太震惊了，我想不到会发生这样的事情。 (This news is really shocking, I didn't think this kind of thing would occur.)
这个公司的发展速度惊人，我想不到他们能取得这么大的成功。 (This company's development is alarmingly fast, I didn't expect they'd be such a large success.)
The above is the most common usage. The speaker is usually emphasizing their own surprise, not the surprise of someone else.
没想到 also has a literal use, where it means something like "didn't think to" or "hadn't thought of" (like 没看到 = "didn't see [sth.]", but with 想 instead of 看):
他没想到去问老师。 (He didn't think to ask the teacher.)
妈妈出门的时候没想到带伞。 (When mother went out, she didn't think to take an umbrella.)
Here, 想 literally means "to think", and the 没 is negating it. This doesn't have the emphasis on surprise, it's just stating facts, so it can be used to describe others.
想不到 also has a literal use, meaning "unable to think"; it's pointing out an inability to do something (like 看不到 = "cannot see [sth.]", but with 想 instead of 看).
A：你能说出澳大利亚的三个动物吗？ (Could you name three Australian animals?)
B：袋鼠、考拉。第三个，想不到。 (Kangaroo, koala. I can't think of a third.)
这个问题的答案很简单，但我想不到。 (This question's answer is so simple, yet I can't think of it.)
大家都想不到办法。 (Nobody can think of a solution.)
For this meaning, you can also use 想不出（来） with the appropriate grammar adjustments.
(PS. Thanks to Sage for help generating example sentences.)