cant log on to Remote DC

Hi gurus.

I have two offices A and B, they are both connected using VPN, In the
past(Imaging that) I have always been able to RDP my remote DC on office b
with no problem but in the last two days, evertime I try to log in I get the
error" the system could not log you on.Username and Password......", The
problem goes away fro a while once the DC on office B is re-booted, then I
can do it all.-

Thanks gurus for any help I can get