Need help! website down after reg transfer


Hi everyone,
I am a newbie with domain setting and everything. I have a website which was developed by a company, however the website is register under their name and recently I have transfer the domain registration under my name. Evertying seems to be ok at first, just follow all the instruction and requested transfer. I didnt make any change what soever until yesterday my website is down and cant be found.

I dont know if this problem is re to DNS setup or anything that I need to set up after.
This transfer is only the domain reg and not the web host. So basically my website is still hosting with different provider.

can anyone help me with this? I tried to change whois info few times, just for the contact info. Im not sure if this could make any problem.

Thank you vy much for your respond.