It’s a pretty complicated subject, with many considerations. I am assuming just your email is going to spam and your website is not affected (because thats a different topic altogether). I am assuming its mainly happening in Gmail. Depending on how fast you want to resolve this and how much experience you have with email tech, you might want to do following steps yourself or hire a company to help out ( yeah there are people in this world whose job is to get people out of spam hell. Pretty crazy don’t you think !! )
1. Setup SPF
2. Setup DKIM
3. Setup Dmarc
4. Email all of your friends ( and probably this group) who are on gmail, and have them find your email on spam and mark it not-spam. Do this for next 2-5 days
5. Going forward do not use your primary domain for sales email.
There are other things you can do including calling Gmail ( asking them for ideas as they are generally helpful) along with a bunch of other routes like using SendGrid for your mail delivery in the interim, however most of the other options are complicated. Unfortunately none of the above steps are guaranteed to fix the problem but its your best bet other than moving out of your domain.