A few days ago my game appeared in the search bar normally. But now he doesn't appear anymore. I added a donation option to it recently. Could this be the cause?
According to the Search & Browse FAQ
"If you're selling your first project, then your page may be placed in our review queue. New sellers are reviewed before their projects are elgible for indexing. This proces may take a few days. Please contact us only after you've waited at least one day."
It looks like this is your first project posted to itch.io, and since you added the option to donate it now needs to be manually reviewed. It'll reappear in search results after someone's made sure it's legit.
Also, there is a specific forum for Questions and Support. Questions like these have already been answered in the past over there, so to get a quick answer I would make sure to search for your question before making a brand new post in the future.
Your game has been quarantined. See above, except it was caught by automated measures, as a safety precaution. It could be due to a change you made recently, I'm not sure. If the situation persists, please contact support.