Sold share in a business not knowing about a big contract in the pipeline

Opened a business with another person.
50//50 shares both directors.
Was bought out for minimal amount has company had no real worth.
Have found out since there was a big deal in the pipeline whilst I was there but only completed when I left.
Is it just a business lesson I have to take on the chin or anything I can do?
Appreciate any advice.

It would be worth a review of your sale agreement, an anti-embarrassment clause is often added to protect you against situations like this. Although if the other Director knew about the deal and was intentionally hiding it from the Board then they would presumably not have signed a sale agreement with a clause like this in there.

I'll let others comment on whether there might be something in company law you can use to make a challenge, do you have clear proof the deal was in the works before you sold and that it was intentionally hidden from you to suppress the price?

There was no such clause.
The board was only the 2 of us.
The only proof was that I asked about a specific deal - was told it was dead in the water (via emails) and then happened as soon as I left.

Once bitten, twice shy
I wouldn't be doing any more deals with your ex co-investor

Unless you have something in writing about future profits when you sold your share, then that's it I'd say

This might be tough to read. Is there any chance that your presence in the business could have been viewed as problematic by the prospective client and your departure was the only way forward?

Just physical items sold so don’t think it was my presence but you could be right.
Anyway nothing I can do.
Lesson I’ll have to learn.