How come I can't go online with my xbox 360?

for like 4-5 days straight i couldn't go online with my xbox 360!and guess what?2-3 weeks ago i had RROD.i got a new one from Microsoft 2 DAYS LATER!pretty quick huh?so anyways,this my 2-3 week old xbox 360,and im already having trouble going online.if you have an xbox 360 and can go online with it,would you please turn it on and see if you can go online,then give me what happened.and dont tell me to call microsoft!but tell me why this is happining and how i can fix it.How come I can't go online with my xbox 360?
If you were having a problem recently, there was an issue with the server that lasted about 4 hours, if you tried to log on then you couldn't, if you've never logged on before, than it's a problem with your router.How come I can't go online with my xbox 360?
Maybe you changed your settings for your connection or they got reset
Did you remember to take out the hard drive when you sent it back to Microsoft? And did you remember to put in your hard drive into your xbox? Ive had the same problem but what I do is i remove the hard drive and put it back in and it works. Just remember to turn on profile selection option. Make sure you tested your internet connection and to see if the wire is connected.