I had to go through this recently because of a spammer that used my site to upload some malware that I didn’t remove for a couple of days.
1. Go to https://www.spamhaus.org/lookup/ and enter in your domain name to see if it has been blocklisted
2. If it is fill out the form and submit the request to remove your site from the blocklist
3. Expect this to take 24-48 hours, during this time contact your website domain registrar and tell them you have submitted your request to be removed from the blocklist
4. In most cases your registrar probably disabled your website from using their name servers; either beg them to let you use them again or add in different ones
5. Make sure to take care of whatever got you listed on the blocklist, you can see why you were blocklisted here: https://urlhaus.abuse.ch/
Then just enjoy your site that will now have some permanent reputation damage for the duration of its existence, and be more careful now that an internet bully can hold your site as hostage whenever they please.