I've had this problem on and off ever since I started using DH. I find a stuck mailbox always starts working between me sending a Support request an
Support say this is a known issue due to databse problems, and they are working to fix it but have no timescale.
If you mean doesn;t respond at all, that happens I find when the panel is taken down for maintenance (an larmingly frequent event) in the middle of my session. Go back to panel.dreamhost.com and you see the outage notice.