Sorry to hear you got no credit for your work.
Here are some considerations:
= Directly contact the artist.
Tell him he violated your agreement by not giving you credit.
You sound like a very reasonable person, so talking in person, or by phone, would work.
If in person, maybe the presence of a neutral third party might help.
If you are likely to get hot-headed, contact him by e-mail or letter, however, it is far less potent, then in person. Have a friend read what you write before hitting “Send.”
Be clear what you want for a remedy: your name on the credits.
(Did you do the titles for the edit?)
= If you get a bad response, collect any proof you worked on the project, such as e-mails, and digital files still on your editing computer; anything that backs up your claim.
= If anyone else knew about your agreement with the artist, get their written deposition.
= Anywhere the video appears you can challenge the credits, however, it would be prudent to have substantial proof.
= Usually, a suit is a last resort, for which you would need to do all of the above, and it will take even more of your time and money, with no more certain results.
Note: It sounds like the artist does not understand that all movie and music production is a collaborative process, and good will is needed for the communities of artists to work together, for present and future projects.
Good luck to you.
Sweet Corn Productions