You have explained in comments to Tor-Einar's answer that the visit that you described in your application but won't be going on anyway is a
cultural, leisure and educational tour to the Netherlands for Iranian scholars and experts who wish to attend in our leisure/cultural tour that is scheduled for May 31st - June 5th, 2018 and will include several visits of cultural points in the Netherlands
That sounds like fancy language for a special-interest tourist visit. The fact that the tour organizer chose to send you something they phrased as an "invitation letter" rather than a plain booking confirmation does not make your planned tourist visit into a business visit.
Your application accurately described something that is a short tourist visit (no matter what your supporting documents called themselves), and you were granted a long multiple-entry visa to go on that visit.
The general rule is that you're allowed to make minor changes of plan, as long as the visit you actually make is still broadly the same as the one you applied for. In your case there are differences that are arguably not quite minor -- most importantly it doesn't sound like your new plans are a package tour -- but it doesn't sound to me like those differences are so large that you can reasonably be suspected of any deceptive intent by having plans for a package tour when you applied and later go on a different tourist trip to the same Schengen country. In particular, you can't be suspected of "consulate shopping", because even if you applied for the later visit you would have done so at the Dutch consulate.
The fact that you were given a multiple-entry visa indicates that you're trusted to arrange new visits with new premises by yourself without having them pre-vetted by a consulate. This ought to mean that you're also free to make changes of the kind you describe here.
What I would do: Go on your now-planned trip. When you're asked your purpose at the border, say something like
I got this visa for a package tour in the beginning of June, but I was prevented from going due to [insert five words about why]. Now I'm going to take a look around on my own instead. I'll be here until next Thursday (or whatever).
Be sure to have concrete plans about your actual visit ready to show at this point, as arriving third-country nationals should always have -- but it's probably not very likely that you will be asked for them.