I have a 1976 No S Proof Eisenhower dollar coin that might be an error coin too. Any idea what it might be worth?
Well a proof is going to have "S" on it. It will also have highly reflective fields and frosty details. Should look something like this pic I got from google images
You're on the internet you would have be able to find out in the time it took you to ask that question. Although see what @hotwheelsearl posted above. And here's an article on the no S proof, which technically shouldn't have an S anyways as they were minted in Philly and were trial pieces. https://www.pcgs.com/News/The-Unique-1976-Philadelphia-Eisenhower-Dollar
The type II should have only been made in san francisco and should have an S on them.And this looks like a no S Type II error coin that made it into circulation.So we will see.
Philadelphia minted 113,318,000 Type 2 Clad Bicentennial Ike's as well as 4,019,000 type 1. Both of those would have no mint mark or 'S' because they were minted in Philadelphia just like your coin. You can choose not to believe me but there is absolutely nothing special about your coin and it's worth a dollar.
With only one known example in existence, the chances that you have a no S proof is honestly very, very low.