Hdmi, dvi No monitor connection

I have a new PC and I am not getting any connection to the monitor I have tried with a hdmi and a dvi cable I don't have the third a d sub. The monitor says no hdmi connection when plugged in, and the same on dvi, PC was running when picked up ,I did not see which cable they were using. Can all three connection types be used at any time or do they need to be switched between them when the PC is on.
Any advice would be great!!