404 Error for wp-admin after changing server

Hi,

I switched servers yesterday and since then whenever I try to go to the wp-admin section to log in it brings up a 404 error.

I have a feeling it may have something to do with permissions. I tried to overwrite my existing ‘wp-admin’ folder with a fresh one from the latest WordPress download, however my FTP was unable to complete this as it said I didn’t have permission to replace around 90 files.

Anyone know what may be the cause?

(my site is my username with .com after it)