It depends on several things.
What time you are trying to play. (are you trying to log in during maintenance)
If not the above then when are you getting the cant log in error? At the initial login for the patcher or at the server screen after you have patched?
If its after the patcher at the server screen then your probably trying to login during maintenance.
If its at the initial login, before the patcher, then you might just be experiencing a network glitch and should probably just try again. You may also want to contact support if this is the case as there may be a problem somewhere between you, your ISP, and Mythic.
Hope that helps.