Being blocked is sort of like being put in a digital penalty box - Twitter makes it impossible to view content from that person, or communicate with them on Twitter through your account.
Specifically, if you've been blocked, you can no longer follow that person or read their tweets. Their tweets won't appear in your search results, either. You also can't tag the person who has blocked you in photos.
In addition, you can't exchange direct messages with the blocker or view any Moments that account has created.
Since there's no way to simply see a list of the people and accounts blocking you, you need to investigate specific accounts that you suspect might be blocking you.
Start Twitter in a web browser or using the app on your phone.
Use the search box to go to the profile of an account you think might be blocking you.
If you are being blocked, you will not be able to see the profile. Instead, you'll see a message that says "You're blocked. You can't see or follow [the blocker's] Tweets."