TITLE: Resources where I can find open problems in number theory along with their level of difficulty
QUESTION [13 upvotes]: I have completed my master's in mathematics a couple of years ago and due to very strong personal and professional reasons I couldn't get admitted to grad school despite having a good academic background.
I have a really good background in number theory. I don't have  guidance of any professor right now and I want to try working on an open problem.

Can you please let me know of resources( websites/ blogs/books) where I can find open problems in number theory to work on along with their estimated level of difficulty ( if possible)?

Thanks!

REPLY [3 votes]: Some open problem collections from my bookmarks:

Number Theory @ Open Problem Garden
List of unsolved problems in number theory @ Wikipedia
Unsolved problems @ MathWorld
Unsolved Problems and Rewards by Clark Kimberling
Number theory problems by Peter J. Cameron
MathPages Wanted List
Unsolved Problem of the Week Archive