This thingy of people stealing servers is so common... I've witnessed about 3 of these incidents... maybe more...

Step 1: Find his/her house.
Step 2: Beat the living crap out of them while they're sleeping.
Step 3: Steal password.
Step 4: ????
Step 5: Profit.

But on a serious note, you could probably recover the password using email, but if its possible to change it then he/she could have changed the email.

i'm guessing you did not setup the server yourself instead you rented one, in that case
you gave him the password and he changed it, what i would suggest doing is

1-if you have an account in the said rented host try to let them send you the password to the email

2-contact the server renter

3-try to reason with the guy who stole the server.

if however you hosted the server yourself, i don't know what do you mean by "Stole the server and changed the password" i'm afraid i cant help you there.

How do you just get a server STOLEN from you?

I'm new here as well, this is actually my first post. Does your server have a domain name, and if so could you change DNS records on the registrar's panel?

If the password was changed to your hosting panel all you have to do is contact their support team and (if they're any good) they'll help you get it back.