After reading this lengthy article on UEFI and the recommendation to disable CSM for dual boot setups I went ahead and did so. Now I can't boot into UEFI-BIOS (normally pressing either Delete or F2 on boot).
A bit more info> Part of the problem may be my misunderstanding of what constitutes a 'dual boot' system.
Right now I have Linux and Windows, but they are each on totally separate SSD's and I only switch between them via the BIOS on boot. So technically I believe I have a single boot setup and shouldn't have messed with CSM.
To make matters worse, I attempted a BIOS firmware reset thru the Windows rescue menu, holding down shift on restart. Not sure what I did but didn't manage any BIOS reset or update and when rebooted get no video signal, just a black screen.
How do I get back into UEFI BIOS?