So I have 3000 members on my site, and I have no idea how active they are using the site. But I would like to bring them back to the site in case they are not active. How many ways out there that can help me achieving this?

Do you not log the access to the site so you KNOW who are active members?

We have a database if that's your question. But how to query is another question to know active users.

without knowing the db schema it would be impossible to be exact but

last login time or date, number of logins etc?

Please do make the rule if they are not logged in for certain time atleast once in a day if not their membership will be canceled and sending periodic mail in reminding them this will make user to come back to your site and be active

thank you

I think you will have to send an email campaign with a nice template and nice offer to all your members, also, track the average time on website and give some entertainment options on the website to make the members stay for a long time.

Daily Updations and implementing new featurtes and functions are the best technique to keep members active

I will recommend you to make an email campaign and send email to those people who are registered with your site.