Yesterday I had a $-10$ reputation due to User Was Removed.Can I know which user was removed and in which answer I lost my reputation?
Thanks for any reputation.
In general, you can't know which user was removed, and which of your posts the removed user voted on. It is not under all circumstances impossible to find out beyond reasonable doubt, but one needs to make the right guess what to look for and where. Even moderators generally can't find that out. Most user removals that lead to reputation changes don't come to our attention at all, or only via the "user was removed" entry in some reputation history we look at (ours or somebody else's).
There is a related question on Meta Stack Exchange: How can I find posts related to lost reputation due to a “user was removed” message?
I will also add a link to somewhat similar question on Meta MathOverflow: Is it possible to know the list of removed users?
On this meta there is also this feature request: Is it possible to know which posts were affected by user removal? Although some objections against this were raised in the comments and the answer posted there.
What I describe here is not completely reliable but one possibility to get at least approximation of what you want is to compare scores (votes) on your posts at some two points before and after the user removal. Of course, this will include also other changes, not only the ones caused by the user removal.
You can find your reputation breakdown at https://math.stackexchange.com/reputation. Some users occasionally save the content of this page. So if you have saved this at some point and if you download it now, you can compare the two files and see what changes are there.
Another possibility is to use the Data Explorer (SEDE). The data in SEDE are updated once a week - Monday 03:00 UTC. So if you download scores of your posts from there soon after the removal and then once again week
For example, you can try some of these queries. You have to include your user id there.1 The results can then be downloaded in the form of a csv file. (Where each row corresponds to one result, so it is relatively easy to compare versions from different dates by various programs.2)
2 Wikipedia: Comparison of file comparison tools