Area-51 R2 No Video

My grandson's Area-51 R2 (i7-5960X) has three GTX 980 with the Dell/Alienware SLI bridge. He has upgraded to Windows 10 and it has been running a few weeks with no issues - monitor is 27" Acer 144Hz G-Sync connected by DP. Suddenly, there is no video signal - not even during boot. I have checked the video cards, SLI bridge, etc., and don't see anything loose, etc. So, I have a few basic questions.

(1) I have seen conflicting info with 3 cards - is the monitor connected to the right-most port (looking from the back) of the top graphics card or bottom video card (he had it connected to top card).

(2) Any ideas for video suddenly disappearing? It ran with fairly good frame rates for several weeks, but we had this problem once before and then it just went away. I have verified that the monitor is OK via independent source.

(3) Unit came with top card connected to PCIe 1 cables (i.e., labeled PCIe1) and bottom to PCIe-3 cables. I assume these are power supplies and it may not matter, but would like some clarity.

Thank you for your help! I'm just grandpa, but like most grandpa's, I've been using PCs since the 80's. I have no experience with Alienware boxes, however.