N prefix before string in Transact-SQL query

The following articles have some good information on the question. The short answer is just that there's a type mismatch between the unicode column and non-unicode string literal you're using. From the KB article, it looks like omitting the N prefix might still work in some cases, but it would depend on the code page and collation settings of the database. That might explain the change in behavior, if you were previously having success with the no-prefix method.

https://support.microsoft.com/en-us/kb/239530

This denotes that the subsequent string is in Unicode (the N actually stands for National language character set). Which means that you are passing an NCHAR, NVARCHAR or NTEXT value, as opposed to CHAR, VARCHAR or TEXT. See Article #2354 for a comparison of these data types.

Unicode is typically used in database applications which are designed to facilitate code pages which extend beyond the English and Western Europe code pages for example Chinese. Unicode is designed so that extended character sets can still "fit" into database columns.

I think the N-prefix tells sql server to treat the string as a unicode value. nvarchar columns are unicode so you should use this syntax when accessing these columns.