This is not uncommon with a few BluRay players that for some odd reason don't detect the 720p HDTV.
Try a factory reset on the player next and be sure it's firmware is current too.
BluRay (current models) are only HDMI so unless you have something from prior to 2010, I'd factory reset the player and try again.
-> Parting idea. I did find one recent immigrant and they were mixing a player from Europe and a TV from USA and that failed.

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