Do the agents login manually when the server is down?
(They won't be logged in automatically when server goes down)
There are many ACD settings to check (f.e. ADS blocks) but the easy way is to check how login into your ACD queue works is to deaquire a TN and try to login on that set, these instructions could be as follows;
1) Lift handset
2) Press key 0 to "Enter Agent ID"
3) Enter the last 4 digits of your DN (to prevent using double login codes)
When, after login, "GRP .... PRIORITY 1" is shown;
4) Press the Not Ready key to be available for calls.